Revert "mm/memory.c: share the i_mmap_rwsem"

Revert "mm/memory.c: share the i_mmap_rwsem"

This reverts commit c8475d144abb1e62958cc5ec281d2a9e161c1946.

There are several[1][2] of bug reports which points to this commit as potential
cause[3].

Let's revert it until we figure out what's going on.

[1] https://lkml.org/lkml/2014/11/14/342
[2] https://lkml.org/lkml/2014/12/22/213
[3] https://lkml.org/lkml/2014/12/9/741

Signed-off-by: Kirill A. Shutemov <[email protected]>
Reported-by: Sasha Levin <[email protected]>
Acked-by: Davidlohr Bueso <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Peter Zijlstra (Intel) <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Srikar Dronamraju <[email protected]>
Cc: Mel Gorman <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>