vfs: Make sys_sync() use fsync_super() (version 4)

It is unnecessarily fragile to have two places (fsync_super() and do_sync())
doing data integrity sync of the filesystem. Alter __fsync_super() to
accommodate needs of both callers and use it. So after this patch
__fsync_super() is the only place where we gather all the calls needed to
properly send all data on a filesystem to disk.

Nice bonus is that we get a complete livelock avoidance and write_supers()
is now only used for periodic writeback of superblocks.

sync_blockdevs() introduced a couple of patches ago is gone now.

[build fixes folded]

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/internal.h

diff --git a/fs/internal.h b/fs/internal.h
index 343a537..dbec3cc 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -25,6 +25,8 @@
 	return sb == blockdev_superblock;
 }
 
+extern int __sync_blockdev(struct block_device *bdev, int wait);
+
 #else
 static inline void bdev_cache_init(void)
 {
@@ -34,6 +36,11 @@
 {
 	return 0;
 }
+
+static inline int __sync_blockdev(struct block_device *bdev, int wait)
+{
+	return 0;
+}
 #endif
 
 /*
@@ -71,12 +78,3 @@
  * file_table.c
  */
 extern void mark_files_ro(struct super_block *);
-
-/*
- * super.c
- */
-#ifdef CONFIG_BLOCK
-extern void sync_blockdevs(void);
-#else
-static inline void sync_blockdevs(void) { }
-#endif