Remove unneeded lock_kernel() in driver/block/loop.c

Signed-off-by: Diego Woitasen <diego@woitasen.com.ar>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>