As root, try:
# find /lib/modules/`uname -r`/kernel/fs -name "*btrfs*"
And see if you get anything like this:
[root@golem4 ~]# find /lib/modules/`uname -r`/kernel/fs -name "*btrfs*"
/lib/modules/4.2.3-200.fc22.x86_64/kernel/fs/btrfs
/lib/modules/4.2.3-200.fc22.x86_64/kernel/fs/btrfs/btrfs.ko.xz
In other words, do you actually have the btrfs module in your kernel
tree? (the "btrfs.ko.xz" bit is the actual module)