ashmem: Fix ashmem_shrink deadlock.

Don't acquire ashmem_mutex in ashmem_shrink if we've somehow recursed into the
shrinker code from within ashmem. Just bail out, avoiding a deadlock. This is
fine, as ashmem cache pruning is advisory anyhow.

Signed-off-by: Robert Love <rlove@google.com>

drivers/staging/android/ashmem.c

@@ -365,7 +365,11 @@ static int ashmem_shrink(struct shrinker *s, struct shrink_control *sc)
 	if (!sc->nr_to_scan)
 		return lru_count;
 
-	mutex_lock(&ashmem_mutex);
+	/* avoid recursing into this code from within ashmem itself */
+	if (!mutex_trylock(&ashmem_mutex)) {
+		return -1;
+	}
+
 	list_for_each_entry_safe(range, next, &ashmem_lru_list, lru) {
 		struct inode *inode = range->asma->file->f_dentry->d_inode;
 		loff_t start = range->pgstart * PAGE_SIZE;