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

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 <[email protected]>
Signed-off-by: Ilya Dryomov <[email protected]>
Reviewed-by: Sage Weil <[email protected]>