repair: allocate and free extent records individually

repair: allocate and free extent records individually

Instead of allocating inode records in chunks and keeping a freelist of them
which gets released to the system memory allocator in one go use plain malloc
and free for them.  The freelist just means adding a global lock instead
of relying on malloc and free which could be implemented lockless.  In
addition smart allocators like tcmalloc have far less overhead than our
chunk and linked list.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>