exofs: Write sbi->s_nextid as part of the Create command

Before when creating a new inode, we'd set the sb->s_dirt flag,
and sometime later the system would write out s_nextid as part
of the sb_info. Also on inode sync we would force the sb sync
as well.

Define the s_nextid as a new partition attribute and set it
every time we create a new object.
At mount we read it from it's new place.

We now never set sb->s_dirt anywhere in exofs. write_super
is actually never called. The call to exofs_write_super from
exofs_put_super is also removed because the VFS always calls
->sync_fs before calling ->put_super twice.

To stay backward-and-forward compatible we also write the old
s_nextid in the super_block object at unmount, and support zero
length attribute on mount.

This also fixes a BUG where in layouts when group_width was not
a divisor of EXOFS_SUPER_ID (0x10000) the s_nextid was not read
from the device it was written to. Because of the sliding window
layout trick, and because the read was always done from the 0
device but the write was done via the raid engine that might slide
the device view. Now we read and write through the raid engine.

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>

fs/exofs/exofs.h

@@ -77,7 +77,7 @@ struct exofs_layout {
  * our extension to the in-memory superblock
  */
 struct exofs_sb_info {
-	struct exofs_fscb s_fscb;		/* Written often, pre-allocate*/
+	struct exofs_sb_stats s_ess;		/* Written often, pre-allocate*/
 	int		s_timeout;		/* timeout for OSD operations */
 	uint64_t	s_nextid;		/* highest object ID used     */
 	uint32_t	s_numfiles;		/* number of files on fs      */
@@ -281,7 +281,7 @@ int exofs_set_link(struct inode *, struct exofs_dir_entry *, struct page *,
 		    struct inode *);
 
 /* super.c               */
-int exofs_sync_fs(struct super_block *sb, int wait);
+int exofs_sbi_write_stats(struct exofs_sb_info *sbi);
 
 /*********************
  * operation vectors *