Btrfs: fix btrfs boot when compiled as built-in

After the change titled "Btrfs: add support for inode properties", if
btrfs was built-in the kernel (i.e. not as a module), it would cause a
kernel panic, as reported recently by Fengguang:

[    2.024722] BUG: unable to handle kernel NULL pointer dereference at           (null)
[    2.027814] IP: [<ffffffff81501594>] crc32c+0xc/0x6b
[    2.028684] PGD 0
[    2.028684] Oops: 0000 [#1] SMP
[    2.028684] Modules linked in:
[    2.028684] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.13.0-rc7-04795-ga7b57c2 #1
[    2.028684] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[    2.028684] task: ffff88000edba100 ti: ffff88000edd6000 task.ti: ffff88000edd6000
[    2.028684] RIP: 0010:[<ffffffff81501594>]  [<ffffffff81501594>] crc32c+0xc/0x6b
[    2.028684] RSP: 0000:ffff88000edd7e58  EFLAGS: 00010246
[    2.028684] RAX: 0000000000000000 RBX: ffffffff82295550 RCX: 0000000000000000
[    2.028684] RDX: 0000000000000011 RSI: ffffffff81efe393 RDI: 00000000fffffffe
[   ...