[XFS] Fix oops in xfs_file_readdir()

When xfs_file_readdir() exactly fills a buffer, it can move it's index
past the end of the buffer and dereference it even though the result of
the dereference is never used. On some platforms this causes an oops.

SGI-PV: 976923
SGI-Modid: xfs-linux-melb:xfs-kern:30458a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>