xfs: fix simple_return.cocci warning in xfs_file_readdir

fs/xfs/xfs_file.c:919:1-6: WARNING: end returns can be simpified and declaration on line 902 can be dropped

 Simplify a trivial if-return sequence.  Possibly combine with a
 preceding function call.
Generated by: scripts/coccinelle/misc/simple_return.cocci

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>