xfs: fix simple_return.cocci warning in xfs_bmse_shift_one

fs/xfs/libxfs/xfs_bmap.c:5591:1-6: WARNING: end returns can be simpified

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

CC: Brian Foster <bfoster@redhat.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>