[CIFS] when mount helper missing fix slash wrong direction in share

Kernel bugzilla bug #9228

If mount helper (mount.cifs) missing, mounts with form like
//10.11.12.13/c$ would not work (only mounts with slash e.g.
//10.11.12.13\\c$ would work) due to problem with slash supposed
to be converted to backslash by the mount helper (which is not
there).

If we fail on converting an IPv4 address in in4_pton then
try to canonicalize the first slash (ie between sharename
and host ip address) if necessary.  If we have to retry
to check for IPv6 address the slash is already converted
if necessary.

Signed-off-by: Steve French <sfrench@us.ibm.com>