mm/khugepaged: minor reorderings in collapse_shmem()

Several cleanups in collapse_shmem(): most of which probably do not
really matter, beyond doing things in a more familiar and reassuring
order.  Simplify the failure gotos in the main loop, and on success
update stats while interrupts still disabled from the last iteration.

Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1811261526400.2275@eggly.anvils
Fixes: f3f0e1d2

 ("khugepaged: add support of collapse for tmpfs/shmem pages")
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: <stable@vger.kernel.org>	[4.8+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>