btrfs: fix bytes_may_use leak in move_existing_remap()

btrfs: fix bytes_may_use leak in move_existing_remap()

If the call to btrfs_reserve_extent() in move_existing_remap() returns a
smaller extent than we asked for, currently we're not undoing the
bytes_may_use change that we made. Fix this by calling
btrfs_space_info_update_bytes_may_use() again for the difference.

Fixes: bbea42dfb91f ("btrfs: move existing remaps before relocating block group")
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Mark Harmstone <mark@harmstone.com>
Signed-off-by: David Sterba <dsterba@suse.com>