xfs: skip always_cow inodes in xfs_reflink_trim_around_shared

xfs_reflink_trim_around_shared tries to find shared blocks in the
refcount btree.  Always_cow inodes don't have that tree, so don't
bother.

For the existing always_cow code this is a minor optimization.  For
the upcoming zoned code that can do COW without the rtreflink code it
avoids triggering a NULL pointer dereference.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>