xfs_repair: reduce rmap bag memory usage when creating refcounts

xfs_repair: reduce rmap bag memory usage when creating refcounts

The algorithm that computes reference count records uses a "bag"
structure to remember the rmap records corresponding to the current
block.  In the previous patch we converted the bag structure to store
actual rmap records instead of pointers to rmap records owned by another
structure as part of preparing for converting this algorithm to use
in-memory rmap btrees.

However, the memory usage of the bag structure is now excessive -- we
only need the physical extent and inode owner information to generate
refcount records and mark inodes that require the reflink flag.  IOWs,
the flags and offset fields are unnecessary.  Create a custom structure
for the bag, which halves its memory usage.

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