mm/shmem.c: don't run init_page_accessed() against an uninitialised pointer

mm/shmem.c: don't run init_page_accessed() against an uninitialised pointer

If shmem_getpage() returned an error then it didn't necessarily initialise
*pagep.  So shmem_write_begin() shouldn't be playing with *pagep in this
situation.

Fixes an oops when "mm: non-atomically mark page accessed during page
cache allocation where possible" (quite reasonably) left *pagep
uninitialized.

Reported-by: Prabhakar Lad <prabhakar.csengg@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Jan Kara <jack@suse.cz>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>