ceph: mount syntax module parameter

Add read-only module parameters for supported mount syntaxes. Primary
user is the user-space mount helper for catching v2 syntax bugs during
testing by cross verifying if the kernel supports v2 syntax on mount
failure.

Signed-off-by: Venky Shankar <vshankar@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>