hfs: simplify a bit checking for R/O

We have the following pattern in 2 places in HFS

if (!RDONLY)
	hfs_mdb_commit();

This patch pushes the RDONLY check down to 'hfs_mdb_commit()'. This will
make the following patches a bit simpler.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/hfs/mdb.c

@@ -260,6 +260,9 @@ void hfs_mdb_commit(struct super_block *sb)
 {
 	struct hfs_mdb *mdb = HFS_SB(sb)->mdb;
 
+	if (sb->s_flags & MS_RDONLY)
+		return;
+
 	lock_buffer(HFS_SB(sb)->mdb_bh);
 	if (test_and_clear_bit(HFS_FLG_MDB_DIRTY, &HFS_SB(sb)->flags)) {
 		/* These parameters may have been modified, so write them back */