exofs: clean up the correct page collection on write error

if ore_write() fails, we would unlock the pages of pcol, which is now
empty, rather than pcol_copy which owns the pages when ore_write() is
called. this means that no pages will actually be unlocked
(pcol.nr_pages == 0) and the writing process (more accurately, the
syncing process) will hang waiting for a writeback notification that
never comes.

moreover, if ore_write() fails, pcol_free() is called for pcol, whereas
pcol_copy is the object owning the ore_io_state, thus leaking the
ore_io_state.

[Boaz]
I have simplified Idan's original patch a bit, everything else still
holds

Signed-off-by: Idan Kedar <idank@tonian.com>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>

fs/exofs/inode.c

@@ -676,8 +676,10 @@ static int write_exec(struct page_collect *pcol)
 	return 0;
 
 err:
-	_unlock_pcol_pages(pcol, ret, WRITE);
-	pcol_free(pcol);
+	if (!pcol_copy) /* Failed before ownership transfer */
+		pcol_copy = pcol;
+	_unlock_pcol_pages(pcol_copy, ret, WRITE);
+	pcol_free(pcol_copy);
 	kfree(pcol_copy);
 
 	return ret;