sysv: Add forgotten superblock lock init for v7 fs

commit 49475555848d396a0c78fb2f8ecceb3f3f263ef1 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>