nsfs: repair kernel-doc for ns_match()

Commit 1e2328e7 ("fs/nsfs.c: Added ns_match") adds the ns_match()
function with a kernel-doc comment, but the ns parameter was referred to
with ns_common.

Hence, ./scripts/kernel-doc -none fs/nsfs.c warns about it.

Adjust the kernel-doc comment for ns_match() for make W=1 happiness.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>