bcachefs: Factor out btree_key_can_insert()

bcachefs: Factor out btree_key_can_insert()

working on getting rid of all the reasons bch2_insert_fixup_extent() can
fail/stop partway, which is needed for other refactorings.

One of the reasons we could have to bail out is if we're splitting a
compressed extent we might need to add to our disk reservation - but we
can check that before actually starting the insert.

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