bcachefs: Introduce a separate journal watermark for copygc

bcachefs: Introduce a separate journal watermark for copygc

Since journal reclaim -> btree key cache flushing may require the
allocation of new btree nodes, it has an implicit dependency on copygc
in order to make forward progress - so we should avoid blocking copygc
unless the journal is really close to full.

This introduces watermarks to replace our single MAY_GET_UNRESERVED bit
in the journal, and adds a watermark for copygc and plumbs it through.

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