CIFS: add ONCE flag for cifs_dbg type

* Since cifs_vfs_error was just using pr_debug_ratelimited like the rest
  of cifs_dbg, move it there too
* Add a ONCE type flag to call the pr_xxx_once() debug function instead
  of the ratelimited ones.

To convert existing printk_once() calls to this we can run:

    perl -i -pE \
      's/printk_once\s*\(([^" \n]+)(.*)/cifs_dbg(VFS|ONCE,$2/g' \
      fs/cifs/*.c

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>