VFS: Make get_filesystem() return the affected filesystem

Make get_filesystem() return a pointer to the filesystem on which it just
got a ref.

Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>