bcachefs: Push c->mark_lock usage down to where it is needed

bcachefs: Push c->mark_lock usage down to where it is needed

This changes the bch2_mark_key() and related paths to take mark lock
where it is needed, instead of taking it in the upper transaction commit
path - by pushing down locking we'll be able to handle fsck errors
locally instead of requiring a separate check in the btree_gc code for
replicas being marked.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>