Btrfs: convert printk to btrfs_ and fix BTRFS prefix

Convert all applicable cases of printk and pr_* to the btrfs_* macros.

Fix all uses of the BTRFS prefix.

Signed-off-by: Frank Holton <fholton@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index f25deb9..ba94b27 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -374,7 +374,7 @@
 	int ret;
 
 	if (len >= BTRFS_LABEL_SIZE) {
-		pr_err("btrfs: unable to set label with more than %d bytes\n",
+		pr_err("BTRFS: unable to set label with more than %d bytes\n",
 		       BTRFS_LABEL_SIZE - 1);
 		return -EINVAL;
 	}