bcachefs: Fall back to slowpath on exact comparison

bcachefs: Fall back to slowpath on exact comparison

This is basically equivalent to the original strategy of falling back to
checking against the original key when the original key and previous key
didn't differ in the required bits - except, now we only fall back when
the search key doesn't differ in the required bits, which ends up being
a bit faster.

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