x86/asm/memcpy_mcsafe: Fix copy_to_user_mcsafe() exception handling

x86/asm/memcpy_mcsafe: Fix copy_to_user_mcsafe() exception handling

All copy_to_user() implementations need to be prepared to handle faults
accessing userspace. The __memcpy_mcsafe() implementation handles both
mmu-faults on the user destination and machine-check-exceptions on the
source buffer. However, the memcpy_mcsafe() wrapper may silently
fallback to memcpy() depending on build options and cpu-capabilities.

Force copy_to_user_mcsafe() to always use __memcpy_mcsafe() when
available, and otherwise disable all of the copy_to_user_mcsafe()
infrastructure when __memcpy_mcsafe() is not available, i.e.
CONFIG_X86_MCE=n.

This fixes crashes of the form:
    run fstests generic/323 at 2018-07-02 12:46:23
    BUG: unable to handle kernel paging request at 00007f0d50001000
    RIP: 0010:__memcpy+0x12/0x20
    [..]
    Call Trace:
     copyout_mcsafe+0x3a/0x50
     _copy_to_iter_mcsafe+0xa1/0x4a0
     ? dax_alive+0x30/0x50
     dax_iomap_actor+0x1f9/0x280
     ? dax_iomap_rw+0x100/0x100
     iomap_apply+0xba/0x130
     ? dax_iomap_rw+0x100/0x100
     dax_iomap_rw+0x95/0x100
     ? dax_iomap_rw+0x100/0x100
     xfs_file_dax_read+0x7b/0x1d0 [xfs]
     xfs_file_read_iter+0xa7/0xc0 [xfs]
     aio_read+0x11c/0x1a0

Reported-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Tested-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Fixes: 8780356ef630 ("x86/asm/memcpy_mcsafe: Define copy_to_iter_mcsafe()")
Link: http://lkml.kernel.org/r/153108277790.37979.1486841789275803399.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>