Btrfs: make btrfs_debug match pr_debug handling related to DEBUG

The kernel macro pr_debug is defined as a empty statement when DEBUG is
not defined. Make btrfs_debug match pr_debug to avoid spamming
the kernel log with debug messages

Signed-off-by: Frank Holton <fholton@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <clm@fb.com>