libmount: fix mount -a EBUSY for cifs

libmount: fix mount -a EBUSY for cifs

fstab:

 //rhel73/myshare/sub/path  /mnt   cifs

after mount in mountinfo:

 #  grep cifs /proc/self/mountinfo
 47 39 0:40 /sub/path /mnt rw,relatime shared:60 - cifs //rhel73/myshare/sub/path ...
            ^^^^^^^^^
or:

 # grep cifs /proc/self/mountinfo
 47 39 0:40 / /mnt rw,relatime shared:60 - cifs //rhel73/myshare/sub/path ...
            ^

That is so since on kernel cifs code, cifs_get_root (which returns the
entry associated with mnt_root) return s_root if
CIFS_MOUNT_USE_PREFIX_PATH is set, no questions asked.

This situation can occurr often on CIFS mounts, as CIFS servers limit
frequently scope of access to the root path.

[kzak@redhat.com: - add more info to the commit message,
                  - clean up variable names]

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1826719
Signed-off-by: Roberto Bergantinos Corpas <rbergant@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>