nilfs2: get rid of sget use for checking if current mount is present

This stops using sget() for checking if an r/w-mount or an r/o-mount
exists on the device.  This elimination uses a back pointer to the
current mount added to nilfs object.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/nilfs2/the_nilfs.h

@@ -50,6 +50,7 @@ enum {
  * @ns_sem: semaphore for shared states
  * @ns_writer_mutex: mutex protecting ns_writer attach/detach
  * @ns_writer_refcount: number of referrers on ns_writer
+ * @ns_current: back pointer to current mount
  * @ns_sbh: buffer heads of on-disk super blocks
  * @ns_sbp: pointers to super block data
  * @ns_sbwtime: previous write time of super blocks
@@ -98,6 +99,8 @@ struct the_nilfs {
 	struct mutex		ns_writer_mutex;
 	atomic_t		ns_writer_refcount;
 
+	struct nilfs_sb_info   *ns_current;
+
 	/*
 	 * used for
 	 * - loading the latest checkpoint exclusively.