new helper: iterate_supers_type()

Call the given function for all superblocks of given type.  Function
gets a superblock (with s_umount locked shared) and (void *) argument
supplied by caller of iterator.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/super.c
Simple merge

diff --cc include/linux/fs.h
Simple merge