btrfs/acl: add noacl check for btrfs_get_acl()

btrfs returned the acl info though it was mounted with "noacl" option.

Steps to reproduce:

 # mkfs.btrfs /dev/sda2
 # mount /dev/sda2 /mnt
 # touch /mnt/file0
 # setfacl -m 'u:root:x,g::x,o::x' /mnt/file0
 # umount /mnt
 # mount /dev/sda2 -o noacl /mnt
 # getfacl /mnt/file0

we add a noacl check for btrfs_get_acl() to fix it.

---
 fs/btrfs/acl.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

index bb35580..b070f58 100644
@@ -37,6 +37,9 @@ static struct posix_acl *btrfs_get_acl(struct inode *inode, int type)
 	char *value = NULL;
 	struct posix_acl *acl;
 
+	if (!IS_POSIXACL(inode))
+		return NULL;
+
 	acl = get_cached_acl(inode, type);
 	if (acl != ACL_NOT_CACHED)
 		return acl;

1.7.0.1