xfs: move btree bulkload record initialization to ->get_record implementations

xfs: move btree bulkload record initialization to ->get_record implementations

Source kernel commit: 6dfeb0c2ecde71d61af77f65eabbdd6ca9315161

When we're performing a bulk load of a btree, move the code that
actually stores the btree record in the new btree block out of the
generic code and into the individual ->get_record implementations.
This is preparation for being able to store multiple records with a
single indirect call.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>