sysv: convert sysv to use the new mount api

Convert the sysv filesystem to use the new mount API.

Tested by mounting some old sysv & v7 images I found in archives;
there are no mount options, and no remount op, so this conversion
is trivial.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Link: https://lore.kernel.org/r/be08b1c1-c6d7-4e82-b457-87116879bdac@redhat.com


Signed-off-by: Christian Brauner <brauner@kernel.org>