bcachefs: bch2_dev_tryget()

bcachefs: bch2_dev_tryget()

Most uses of bch2_dev_bkey_exists() are going away, where we assume that
because a key references a device the device most exist - instead, we'll
be explicitly checking if the device exists and getting a reference to
it.

This adds the new helpers.

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