bcachefs: member helper cleanups

Some renaming for better consistency

bch2_member_exists	-> bch2_member_alive
bch2_dev_exists		-> bch2_member_exists
bch2_dev_exsits2	-> bch2_dev_exists
bch_dev_locked		-> bch2_dev_locked
bch_dev_bkey_exists	-> bch2_dev_bkey_exists

new helper - bch2_dev_safe

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