syscalls/umount01: Convert to tst_acquire_device()

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
syscalls/umount02: Cleanup.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
syscalls/umount02: Convert to tst_acquire_device()

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
syscalls/umount03: Cleanup.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
syscalls/umount03: Convert to tst_acquire_device()

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>