cifs: Fix creating and resolving absolute NT-style symlinks

cifs: Fix creating and resolving absolute NT-style symlinks

If the SMB symlink is stored on NT server in absolute form then it points
to the NT object hierarchy, which is different from POSIX one and needs
some conversion / mapping.

To make interoperability with Windows SMB server and WSL subsystem, reuse
its logic of mapping between NT paths and POSIX paths into Linux SMB
client.

WSL subsystem on Windows uses for -t drvfs mount option -o symlinkroot=
which specifies the POSIX path where are expected to be mounted lowercase
Windows drive letters (without colon).

Do same for Linux SMB client and add a new mount option -o symlinkroot=
which mimics the drvfs mount option of the same name. It specifies where in
the Linux VFS hierarchy is the root of the DOS / Windows drive letters, and
translates between absolute NT-style symlinks and absolute Linux VFS
symlinks. Default value of symlinkroot is "/mnt", same what is using WSL.

Note that DOS / Windows drive letter symlinks are just subset of all
possible NT-style symlinks. Drive letters live in NT subtree \??\ and
important details about NT paths and object hierarchy are in the comments
in this change.

When symlink target location from non-POSIX SMB server is in absolute form
(indicated by absence of SYMLINK_FLAG_RELATIVE) then it is converted to
Linux absolute symlink according to symlinkroot configuration.

And when creating a new symlink on non-POSIX SMB server in absolute form
then Linux absolute target is converted to NT-style according to
symlinkroot configuration.

When SMB server is POSIX, then this change does not affect neither reading
target location of symlink, nor creating a new symlink. It is expected that
POSIX SMB server works with POSIX paths where the absolute root is /.

This change improves interoperability of absolute SMB symlinks with Windows
SMB servers.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>