btrfs: move ref-verify under CONFIG_BTRFS_DEBUG

Remove CONFIG_BTRFS_FS_REF_VERIFY Kconfig and add it as part of
CONFIG_BTRFS_DEBUG. This should not be impactful to the performance
of debug. The struct btrfs_ref takes an additional u64, btrfs_fs_info
takes an additional spinlock_t and rb_root. All of the ref_verify logic
is still protected by a mount option.

Signed-off-by: Leo Martins <loemra.dev@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>