btrfs: use refcount_t type for the extent buffer reference counter

Instead of using a bare atomic, use the refcount_t type, which despite
being a structure that contains only an atomic, has an API that checks
for underflows and other hazards. This doesn't change the size of the
extent_buffer structure.

This removes the need to do things like this:

    WARN_ON(atomic_read(&eb->refs) == 0);
    if (atomic_dec_and_test(&eb->refs)) {
        (...)
    }

And do just:

    if (refcount_dec_and_test(&eb->refs)) {
        (...)
    }

Since refcount_dec_and_test() already triggers a warning when we decrement
a ref count that has a value of 0 (or below zero).

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>