sysv: Add forgotten superblock lock init for v7 fs

Superblock lock was replaced with (un)lock_super() removal, but left
uninitialized for Seventh Edition UNIX filesystem in the following commit (3.7):
c07cb01c sysv: drop lock/unlock super
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/sysv/super.c

@@ -487,6 +487,7 @@ static int v7_fill_super(struct super_block *sb, void *data, int silent)
 	sbi->s_sb = sb;
 	sbi->s_block_base = 0;
 	sbi->s_type = FSTYPE_V7;
+	mutex_init(&sbi->s_lock);
 	sb->s_fs_info = sbi;
 	
 	sb_set_blocksize(sb, 512);