btrfs: assert nowait mode is not used for some btree search functions

Adds nowait asserts to btree search functions which are not used by
buffered IO and direct IO paths.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Stefan Roesch <shr@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>