Hi,

I have some code to use the new mount API from user space.

The kernel changes are just making the code use the fs_context logging
feature.

The sample userspace prog (fsopen.c) is just a PoC showing how mounting
is done and how the mount errors are read.

If you change the prog to use a wrong version for example (vers=4.0) you
get this:

    $ gcc -o fsopen fsopen.c
    $ ./fsopen
    fsconfig(sfd, FSCONFIG_SET_STRING, "vers", "4.0", 0): Invalid argument
    kernel mount errors:
    e Unknown vers= option specified: 4.0

There are some cons to using this API, mainly that mount.cifs will have
to encode more knowledge and processing of the user-provided option
before passing it to the kernel.

Where previously we would pass most options as-is to the kernel, making
it easier to add new ones without patching mount.cifs; we know have to
know if the option is a key=string, or key=int, or boolean (key/nokey)
and call the appropriate FSCONFIG_SET_{STRING,FLAG,PATH,...}.

The pros are that we process one option at a time and we can fail early
with verbose, helpful messages to the user.