xfs: more reserved blocks fixups

This mangles the reserved blocks counts a little more.

1) add a helper function for the default reserved count
2) add helper functions to save/restore counts on ro/rw
3) save/restore reserved blocks on freeze/thaw
4) disallow changing reserved count while readonly

V2: changed field name to match Dave's changes

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Alex Elder <aelder@sgi.com>

fs/xfs/xfs_mount.h

@@ -421,6 +421,7 @@ typedef struct xfs_mod_sb {
 } xfs_mod_sb_t;
 
 extern int	xfs_log_sbcount(xfs_mount_t *, uint);
+extern __uint64_t xfs_default_resblks(xfs_mount_t *mp);
 extern int	xfs_mountfs(xfs_mount_t *mp);
 
 extern void	xfs_unmountfs(xfs_mount_t *);