checkpatch: warn when declaring "struct spinlock foo;"

spinlock_t should always be used.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: "Luis R. Rodriguez" <mcgrof@do-not-panic.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>