xfs: simplify if-else condition in xfs_reflink_trim_around_shared

"else" is not generally useful after a return,
so remove it for clean code.

There is no logical changes.

Signed-off-by: Zeng Heng <zengheng4@huawei.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <david@fromorbit.com>