btrfs: fix typo in space info explanation

There's an explanation of how space info works at the top of
fs/btrfs/space-info.c, which makes reference to a variable called
bytes_may_reserve.  There's nothing called that in the code, and wasn't
at time the comment was written; as far I can tell this is a typo, and
it should actually be bytes_may_use.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Mark Harmstone <maharmstone@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>