xfs_io: provide long-format help for falloc

xfs_io: provide long-format help for falloc

Provide long-format help for falloc so that users can learn about
the command.

Note for xfstest writers: If you need to check that a particular
fallocate mode works (-c/-i/-p/-u) on a given filesystem, you should
call _require_xfs_io_command with the falloc subcommand directly, (i.e.
_require_xfs_io_command funshare) because the subcommands are
special-cased to actually try the command.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>