vfs: Introduce the basic header for the new mount API's filesystem context

vfs: Introduce the basic header for the new mount API's filesystem context

Introduce a filesystem context concept to be used during superblock
creation for mount and superblock reconfiguration for remount.  This is
allocated at the beginning of the mount procedure and into it is placed:

 (1) Filesystem type.

 (2) Namespaces.

 (3) Source/Device names (there may be multiple).

 (4) Superblock flags (SB_*).

 (5) Security details.

 (6) Filesystem-specific data, as set by the mount options.

Also introduce a struct for typed key=value parameter concept with which
configuration data will be passed to filesystems.  This will allow not only
for ordinary string values, but also make it possible to pass more exotic
values such as binary blobs, paths and fds with greater ease.

Signed-off-by: David Howells <dhowells@redhat.com>