xfs_repair: bulk load records into new btree blocks

Amortize the cost of indirect calls further by loading a batch of
records into a new btree block instead of one record per ->get_record
call.  On a rmap btree with 3.9 million records, this reduces the
runtime of xfs_btree_bload by 3% for xfsprogs.  For the upcoming online
repair functionality, this will reduce runtime by 6% when spectre
mitigations are enabled in the kernel.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>