bcachefs: Avoid write lock on mark_lock

bcachefs: Avoid write lock on mark_lock

mark_lock is a frequently taken lock, and there's also potential for
deadlocks since currently bch2_clear_page_bits which is called from
memory reclaim has to take it to drop disk reservations.

The disk reservation get path takes it when it recalculates the number
of sectors known to be available, but it's not really needed for
consistency.  We just want to make sure we only have one thread updating
the sectors_available count, which we can do with a dedicated mutex.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>