xfs: cleanup xfs_bmse_shift_one goto mess

xfs_bmse_shift_one() jumps around determining whether to shift or
merge, making the code flow difficult to follow. Clean it up and
use direct error returns (including XFS_WANT_CORRUPTED_RETURN) to
make the code flow better and be easier to read.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>