cifs: mount with "unix" mount option for SMB1 incorrectly handled

Although by default we negotiate CIFS Unix Extensions for SMB1 mounts to
Samba (and they work if the user does not specify "unix" or "posix" or
"linux" on mount), and we do properly handle when a user turns them off
with "nounix" mount parm.  But with the changes to the mount API we
broke cases where the user explicitly specifies the "unix" option (or
equivalently "linux" or "posix") on mount with vers=1.0 to Samba or other
servers which support the CIFS Unix Extensions.

 "mount error(95): Operation not supported"

and logged:

 "CIFS: VFS: Check vers= mount option. SMB3.11 disabled but required for POSIX extensions"

even though CIFS Unix Extensions are supported for vers=1.0  This patch fixes
the case where the user specifies both "unix" (or equivalently "posix" or
"linux") and "vers=1.0" on mount to a server which supports the
CIFS Unix Extensions.

Cc: stable@vger.kernel.org
Reviewed-by: David Howells <dhowell@redhat.com>
Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>