nilfs2: add helper to get if volume is in a valid state

This adds a helper function, nilfs_valid_fs() which returns if nilfs
is in a valid state or not.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
diff --git a/fs/nilfs2/the_nilfs.c b/fs/nilfs2/the_nilfs.c
index aea2b58..890a8d3 100644
--- a/fs/nilfs2/the_nilfs.c
+++ b/fs/nilfs2/the_nilfs.c
@@ -261,16 +261,12 @@
 	struct nilfs_recovery_info ri;
 	unsigned int s_flags = sbi->s_super->s_flags;
 	int really_read_only = bdev_read_only(nilfs->ns_bdev);
-	unsigned valid_fs;
+	int valid_fs = nilfs_valid_fs(nilfs);
 	int err;
 
 	if (nilfs_loaded(nilfs))
 		return 0;
 
-	down_write(&nilfs->ns_sem);
-	valid_fs = (nilfs->ns_mount_state & NILFS_VALID_FS);
-	up_write(&nilfs->ns_sem);
-
 	if (!valid_fs) {
 		printk(KERN_WARNING "NILFS warning: mounting unchecked fs\n");
 		if (s_flags & MS_RDONLY) {