Artful update to v4.12.2 stable release

SRU Justification

    Impact:
       The upstream process for stable tree updates is quite similar
       in scope to the Ubuntu SRU process, e.g., each patch has to
       demonstrably fix a bug, and each patch is vetted by upstream
       by originating either directly from a mainline/stable Linux tree or
       a minimally backported form of that patch. The v4.12.2 upstream stable
       patch set is now available. It should be included in the Ubuntu
       kernel as well.

       git://git.kernel.org/

    TEST CASE: TBD

       The following patches from the v4.12.2 stable release shall be applied:

mqueue: fix a use-after-free in sys_mq_notify()
imx-serial: RX DMA startup latency
proc: Fix proc_sys_prune_dcache to hold a sb reference
locking/rwsem-spinlock: Fix EINTR branch in __down_write_common()
staging: vt6556: vnt_start Fix missing call to vnt_key_init_table.
staging: comedi: fix clean-up of comedi_class in comedi_init()
crypto: caam - fix gfp allocation flags (part I)
crypto: rsa-pkcs1pad - use constant time memory comparison for MACs
ext4: check return value of kstrtoull correctly in reserved_clusters_store
x86/mm/pat: Don't report PAT on CPUs that don't support it
Linux 4.12.2