mm/vmscan.c: make may_enter_fs bool in shrink_page_list()

This gives some size improvement:

$size mm/vmscan.o (before)
   text	   data	    bss	    dec	    hex	filename
  53670	  24123	     12	  77805	  12fed	mm/vmscan.o

$size mm/vmscan.o (after)
   text	   data	    bss	    dec	    hex	filename
  53648	  24123	     12	  77783	  12fd7	mm/vmscan.o

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/Message-ID:
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>