xfs: prepare repair for bulk loading

Before we start merging the online repair functions, let's improve the
bulk loading code a bit.  First, we need to fix a misinteraction between
the AIL and the btree bulkloader wherein the delwri at the end of the
bulk load fails to queue a buffer for writeback if it happens to be on
the AIL list.

Second, we introduce EFIs in the btree bulkloader block allocator to to
guarantee that staging blocks are freed if the filesystem goes down
before committing the new btree.

Third, we change the bulkloader itself to copy multiple records into a
block if possible, and add some debugging knobs so that developers can
control the slack factors, just like they can do for xfs_repair.

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