shmem: make shmem_inode_info::lock irq-safe

We are going to need to call shmem_charge() under tree_lock to get
accoutning right on collapse of small tmpfs pages into a huge one.

The problem is that tree_lock is irq-safe and lockdep is not happy, that
we take irq-unsafe lock under irq-safe[1].

Let's convert the lock to irq-safe.

[1] https://gist.github.com/kiryl/80c0149e03ed35dfaf26628b8e03cdbc

Link: http://lkml.kernel.org/r/1466021202-61880-34-git-send-email-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/shm.c

@@ -766,10 +766,10 @@ static void shm_add_rss_swap(struct shmid_kernel *shp,
 	} else {
 #ifdef CONFIG_SHMEM
 		struct shmem_inode_info *info = SHMEM_I(inode);
-		spin_lock(&info->lock);
+		spin_lock_irq(&info->lock);
 		*rss_add += inode->i_mapping->nrpages;
 		*swp_add += info->swapped;
-		spin_unlock(&info->lock);
+		spin_unlock_irq(&info->lock);
 #else
 		*rss_add += inode->i_mapping->nrpages;
 #endif