bcachefs: Change bch2_dev_lookup() to not use lookup_bdev()

bcachefs: Change bch2_dev_lookup() to not use lookup_bdev()

bch2_dev_lookup() is used from the extended attribute set methods, for
setting the target options, where we're already holding an inode lock -
it turns out pathname lookups also take inode locks, so that was
susceptible to deadlocks.

Fortunately we already stash the device name in ca->name. This does
change user-visible behaviour though: instead of specifying e.g.
/dev/sda1, user must now specify sda1.

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