mm: introduce do_shared_fault() and drop do_fault()

Introduce do_shared_fault().  The function does what do_fault() does for
write faults to shared mappings

Unlike do_fault(), do_shared_fault() is relatively clean and
straight-forward.

Old do_fault() is not needed anymore.  Let it die.

[lliubbo@gmail.com: fix NULL pointer dereference]
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Matthew Wilcox <matthew.r.wilcox@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>