powerpc/crypto/crc32c-vpmsum: Fix missing preempt_disable()

powerpc/crypto/crc32c-vpmsum: Fix missing preempt_disable()

In crc32c_vpmsum() we call enable_kernel_altivec() without first
disabling preemption, which is not allowed:

  WARNING: CPU: 9 PID: 2949 at ../arch/powerpc/kernel/process.c:277 enable_kernel_altivec+0x100/0x120
  Modules linked in: dm_thin_pool dm_persistent_data dm_bio_prison dm_bufio libcrc32c vmx_crypto ...
  CPU: 9 PID: 2949 Comm: docker Not tainted 4.11.0-rc5-compiler_gcc-6.3.1-00033-g308ac7563944 #381
  ...
  NIP [c00000000001e320] enable_kernel_altivec+0x100/0x120
  LR [d000000003df0910] crc32c_vpmsum+0x108/0x150 [crc32c_vpmsum]
  Call Trace:
    0xc138fd09 (unreliable)
    crc32c_vpmsum+0x108/0x150 [crc32c_vpmsum]
    crc32c_vpmsum_update+0x3c/0x60 [crc32c_vpmsum]
    crypto_shash_update+0x88/0x1c0
    crc32c+0x64/0x90 [libcrc32c]
    dm_bm_checksum+0x48/0x80 [dm_persistent_data]
    sb_check+0x84/0x120 [dm_thin_pool]
    dm_bm_validate_buffer.isra.0+0xc0/0x1b0 [dm_persistent_data]
    dm_bm_read_lock+0x80/0xf0 [dm_persistent_data]
    __create_persistent_data_objects+0x16c/0x810 [dm_thin_pool]
    dm_pool_metadata_open+0xb0/0x1a0 [dm_thin_pool]
    pool_ctr+0x4cc/0xb60 [dm_thin_pool]
    dm_table_add_target+0x16c/0x3c0
    table_load+0x184/0x400
    ctl_ioctl+0x2f0/0x560
    dm_ctl_ioctl+0x38/0x50
    do_vfs_ioctl+0xd8/0x920
    SyS_ioctl+0x68/0xc0
    system_call+0x38/0xfc

It used to be sufficient just to call pagefault_disable(), because that
also disabled preemption. But the two were decoupled in commit 8222dbe21e79
("sched/preempt, mm/fault: Decouple preemption from the page fault
logic") in mid 2015.

So add the missing preempt_disable/enable(). We should also call
disable_kernel_fp(), although it does nothing by default, there is a
debug switch to make it active and all enables should be paired with
disables.

Fixes: 6dd7a82cc54e ("crypto: powerpc - Add POWER8 optimised crc32c")
Cc: stable@vger.kernel.org # v4.8+
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>