bio integrity: do not assume bio_integrity_pool exists if bioset exists

bio integrity: do not assume bio_integrity_pool exists if bioset exists

bio_integrity_alloc() and bio_integrity_free() assume that if a bio was
allocated from a bioset that that bioset also had its bio_integrity_pool
allocated using bioset_integrity_create().  This is a very bad
assumption given that bioset_create() and bioset_integrity_create() are
completely disjoint.  Not all callers of bioset_create() have been
trained to also call bioset_integrity_create() -- and they may not care
to be.

Fix this by falling back to kmalloc'ing 'struct bio_integrity_payload'
rather than force all bioset consumers to (wastefully) preallocate a
bio_integrity_pool that they very likely won't actually need (given the
niche nature of the current block integrity support).

Otherwise, a NULL pointer "Kernel BUG" with a trace like the following
will be observed (as seen on s390x using zfcp storage) because dm-io
doesn't use bioset_integrity_create() when creating its bioset:

    [  791.643338] Call Trace:
    [  791.643339] ([<00000003df98b848>] 0x3df98b848)
    [  791.643341]  [<00000000002c5de8>] bio_integrity_alloc+0x48/0xf8
    [  791.643348]  [<00000000002c6486>] bio_integrity_prep+0xae/0x2f0
    [  791.643349]  [<0000000000371e38>] blk_queue_bio+0x1c8/0x3d8
    [  791.643355]  [<000000000036f8d0>] generic_make_request+0xc0/0x100
    [  791.643357]  [<000000000036f9b2>] submit_bio+0xa2/0x198
    [  791.643406]  [<000003ff801f9774>] dispatch_io+0x15c/0x3b0 [dm_mod]
    [  791.643419]  [<000003ff801f9b3e>] dm_io+0x176/0x2f0 [dm_mod]
    [  791.643423]  [<000003ff8074b28a>] do_reads+0x13a/0x1a8 [dm_mirror]
    [  791.643425]  [<000003ff8074b43a>] do_mirror+0x142/0x298 [dm_mirror]
    [  791.643428]  [<0000000000154fca>] process_one_work+0x18a/0x3f8
    [  791.643432]  [<000000000015598a>] worker_thread+0x132/0x3b0
    [  791.643435]  [<000000000015d49a>] kthread+0xd2/0xd8
    [  791.643438]  [<00000000005bc0ca>] kernel_thread_starter+0x6/0xc
    [  791.643446]  [<00000000005bc0c4>] kernel_thread_starter+0x0/0xc

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@fb.com>