ei_update_lock is largely vestigal and will probably be removed, but
we're not ready for that just yet.

this fixes some lockdep splats with the new lockdep support for btree
node locks; they're harmless, since we were taking ei_update_lock before
actually locking any btree nodes, but "any btree nodes locked" are now
tracked at the btree_trans level.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>