ceph: fix dout() compile warnings in ceph_filemap_fault()

PAGE_CACHE_SIZE is unsigned long on all architectures, however size_t
is either unsigned int or unsigned long.  Rather than change format
strings, cast PAGE_CACHE_SIZE to size_t to be in line with dout()s in
ceph_page_mkwrite().

Cc: Yan, Zheng <zheng.z.yan@intel.com>
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>