get rid of restarts in sync_filesystems()

At the same time we can kill s_need_restart and local mutex in there.
__put_super() made public for a while; will be gone later.

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

diff --cc fs/super.c
Simple merge

diff --cc fs/sync.c
Simple merge

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