pstore: Convert "psinfo" locking to mutex

Currently pstore can only have a single backend attached at a time, and it
tracks the active backend via "psinfo", under a lock. The locking for this
does not need to be a spinlock, and in order to avoid may_sleep() issues
during future changes to pstore_unregister(), switch to a mutex instead.

Link: https://lore.kernel.org/lkml/20200506152114.50375-4-keescook@chromium.org/Signed-off-by: Kees Cook <keescook@chromium.org>

fs/pstore/platform.c

@@ -72,7 +72,7 @@ static DECLARE_WORK(pstore_work, pstore_dowork);
  * psinfo_lock just protects "psinfo" during
  * calls to pstore_register()
  */
-static DEFINE_SPINLOCK(psinfo_lock);
+static DEFINE_MUTEX(psinfo_lock);
 struct pstore_info *psinfo;
 
 static char *backend;
@@ -574,11 +574,11 @@ int pstore_register(struct pstore_info *psi)
 		return -EINVAL;
 	}
 
-	spin_lock(&psinfo_lock);
+	mutex_lock(&psinfo_lock);
 	if (psinfo) {
 		pr_warn("backend '%s' already loaded: ignoring '%s'\n",
 			psinfo->name, psi->name);
-		spin_unlock(&psinfo_lock);
+		mutex_unlock(&psinfo_lock);
 		return -EBUSY;
 	}
 
@@ -587,7 +587,7 @@ int pstore_register(struct pstore_info *psi)
 	psinfo = psi;
 	mutex_init(&psinfo->read_mutex);
 	sema_init(&psinfo->buf_lock, 1);
-	spin_unlock(&psinfo_lock);
+	mutex_unlock(&psinfo_lock);
 
 
 	if (psi->flags & PSTORE_FLAGS_DMESG)