cifs: Add support for creating WSL-style symlinks

This change implements support for creating new symlink in WSL-style by
Linux cifs client when -o reparse=wsl mount option is specified. WSL-style
symlink uses reparse point with tag IO_REPARSE_TAG_LX_SYMLINK and symlink
target location is stored in reparse buffer in UTF-8 encoding prefixed by
32-bit flags. Flags bits are unknown, but it was observed that WSL always
sets flags to value 0x02000000. Do same in Linux cifs client.

New symlinks would be created in WSL-style only in case the mount option
-o reparse=wsl is specified, which is not by default. So default CIFS
mounts are not affected by this change.

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