9p: rename p9_free_req() function

In sight of the next patch to add a refcount in p9_req_t, rename
the p9_free_req() function in p9_release_req().

In the next patch the actual kfree will be moved to another function.

Link: http://lkml.kernel.org/r/20180811144254.23665-1-tomasbortoli@gmail.com
Signed-off-by: Tomas Bortoli <tomasbortoli@gmail.com>
Acked-by: Jun Piao <piaojun@huawei.com>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>