ext4: only initialize mmp_bdevname once

mmp_bdevname is currently both initialized nested inside the kthread_run
call in ext4_multi_mount_protect and in the kmmpd thread started by it.

Lift the initiaization out of the kthread_run call in
ext4_multi_mount_protect, move the BUILD_BUG_ON next to it and remove
the duplicate assignment inside of kmmpd.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20220713055317.1888500-9-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/ext4/mmp.c

@@ -150,8 +150,6 @@ static int kmmpd(void *data)
 	mmp_check_interval = max(EXT4_MMP_CHECK_MULT * mmp_update_interval,
 				 EXT4_MMP_MIN_CHECK_INTERVAL);
 	mmp->mmp_check_interval = cpu_to_le16(mmp_check_interval);
-	BUILD_BUG_ON(sizeof(mmp->mmp_bdevname) < BDEVNAME_SIZE);
-	bdevname(bh->b_bdev, mmp->mmp_bdevname);
 
 	memcpy(mmp->mmp_nodename, init_utsname()->nodename,
 	       sizeof(mmp->mmp_nodename));
@@ -372,13 +370,15 @@ int ext4_multi_mount_protect(struct super_block *sb,
 
 	EXT4_SB(sb)->s_mmp_bh = bh;
 
+	BUILD_BUG_ON(sizeof(mmp->mmp_bdevname) < BDEVNAME_SIZE);
+	bdevname(bh->b_bdev, mmp->mmp_bdevname);
+
 	/*
 	 * Start a kernel thread to update the MMP block periodically.
 	 */
 	EXT4_SB(sb)->s_mmp_tsk = kthread_run(kmmpd, sb, "kmmpd-%.*s",
 					     (int)sizeof(mmp->mmp_bdevname),
-					     bdevname(bh->b_bdev,
-						      mmp->mmp_bdevname));
+					     mmp->mmp_bdevname);
 	if (IS_ERR(EXT4_SB(sb)->s_mmp_tsk)) {
 		EXT4_SB(sb)->s_mmp_tsk = NULL;
 		ext4_warning(sb, "Unable to create kmmpd thread for %s.",