xfs: cleanup xfs_bmse_merge returns

Signed-off-by: Dave Chinner <dchinner@redhat.com>

xfs_bmse_merge() has a jump label for return that just returns the
error value. Convert all the code to just return the error directly
and use XFS_WANT_CORRUPTED_RETURN. This also allows the final call
to xfs_bmbt_update() to return directly.

Noticed while reviewing coccinelle return cleanup patches and
wondering why the same return pattern as in xfs_bmse_shift_one()
wasn't picked up by the checker pattern...

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