ceph: Avoid data inconsistency due to d-cache aliasing in readpage()

If the length of data to be read in readpage() is exactly
PAGE_CACHE_SIZE, the original code does not flush d-cache
for data consistency after finishing reading. This patches fixes
this.

Signed-off-by: Li Wang <liwang@ubuntukylin.com>
Signed-off-by: Sage Weil <sage@inktank.com>