mm, dax: fix DAX deadlocks

The following two locking commits in the DAX code:

commit 84317297 ("dax: fix race between simultaneous faults")
commit 46c043ed ("mm: take i_mmap_lock in unmap_mapping_range() for DAX")

introduced a number of deadlocks and other issues which need to be fixed
for the v4.3 kernel.  The list of issues in DAX after these commits
(some newly introduced by the commits, some preexisting) can be found
here:

  https://lkml.org/lkml/2015/9/25/602

 (Subject: "Re: [PATCH] dax: fix deadlock in __dax_fault").

This undoes most of the changes introduced by those two commits,
essentially returning us to the DAX locking scheme that was used in
v4.2.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Dan Williams <dan.j.williams@intel.com>
Tested-by: Dave Chinner <dchinner@redhat.com>
Cc: Jan Kara <jack@suse.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <matthew.r.wilcox@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>