btrfs: fix bytes_may_use leak in do_remap_reloc_trans()

btrfs: fix bytes_may_use leak in do_remap_reloc_trans()

If the call to btrfs_reserve_extent() in do_remap_reloc_trans() 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: fd6594b1446c ("btrfs: replace identity remaps with actual remaps when doing relocations")
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Mark Harmstone <mark@harmstone.com>
Signed-off-by: David Sterba <dsterba@suse.com>