[LibOS] Add stat() and hstat() callbacks to FIFOs

[dimakuv]

Description of the changes

Previously, FIFO pseudo-files (named pipes) did not have callbacks of stat() (used on FIFO filenames) and hstat() (used on FIFO handles/FDs). This breaks some apps that want to double-check that the file they have opened or want to open is a FIFO, using the stat() or fstat() syscalls. There was no good reason to skip those callbacks, so this commit adds their dummy implementations.

Fixes #1896.

How to test this PR?

Added some sub-tests in LibOS, and activated read03 in LTP.

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This change is 

[mkow]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions, not enough approvals from maintainers (2 more required), not enough approvals from different teams (1 more required, approved so far: Intel) (waiting on @dimakuv)

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libos/test/regression/mkfifo.c line 23 at r1 (raw file):

    }

    struct stat buf;

I really dislike having buf and buffer in a single function at the same time.

Suggestion:

struct stat stat_buf;

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libos/test/regression/mkfifo.c line 46 at r1 (raw file):

        }

        struct stat buf;

ditto, OTOH, you already have a buffer which you could use here, why another one?

[dimakuv]

Reviewable status: 2 of 3 files reviewed, 2 unresolved discussions, not enough approvals from maintainers (2 more required), not enough approvals from different teams (1 more required, approved so far: Intel), "fixup! " found in commit messages' one-liners (waiting on @mkow)

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libos/test/regression/mkfifo.c line 23 at r1 (raw file):

Previously, mkow (Michał Kowalczyk) wrote…

I really dislike having buf and buffer in a single function at the same time.

Done.

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libos/test/regression/mkfifo.c line 46 at r1 (raw file):

Previously, mkow (Michał Kowalczyk) wrote…

ditto, OTOH, you already have a buffer which you could use here, why another one?

Done.

[mkow]

Reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: all files reviewed, all discussions resolved, not enough approvals from maintainers (1 more required), "fixup! " found in commit messages' one-liners

[kailun-qin]

Reviewed 2 of 3 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: all files reviewed, all discussions resolved, "fixup! " found in commit messages' one-liners

[mkow]

Reviewed all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved

[kailun-qin]

Reviewed all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved