bcachefs: Use ei_update_lock consistently

bcachefs: Use ei_update_lock consistently

This is prep work for using deferred btree updates for inode updates -
the way inodes are done now we're relying on btree locking for ei_inode
and ei_update_lock could probably be removed, but it'll actually be
needed when we switch to deferred updates.

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