xfs: refactor xfs_reflink_find_shared

xfs: refactor xfs_reflink_find_shared

Move lookup of the perag structure from the callers into the helpers,
and return the offset into the extent of the shared region instead of
the block number that needs post-processing.  This prepares the
callsites for the creation of an rt-specific variant in the next patch.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
[djwong: port to the middle of the rtreflink series for cleanliness]
Signed-off-by: Darrick J. Wong <djwong@kernel.org>