[PATCH] shmem: remove info->sem

Between inode->i_sem and info->lock comes info->sem; but it doesn't
guard thoroughly against the difficult races (truncate during read),
and serializes reads from tmpfs unlike other filesystems.  I'd prefer
to work with just i_sem and info->lock, backtracking when necessary
(when another task allocates block or metablock at the same time).

(I am not satisfied with the locked setting of next_index at the start
of shmem_getpage_locked: it's one lock hold too many, and it doesn't
really fix races against truncate better than before: another patch in
a later batch will resolve that.)

include/linux/shmem_fs.h

@@ -11,7 +11,6 @@ extern atomic_t shmem_nrpages;
 
 struct shmem_inode_info {
 	spinlock_t		lock;
-	struct semaphore 	sem;
 	unsigned long		next_index;
 	swp_entry_t		i_direct[SHMEM_NR_DIRECT]; /* for the first blocks */
 	void		      **i_indirect; /* indirect blocks */