FileStore: Round offset of fiemap down aligned with CEPH_PAGE_SIZE.

There is a bug on xfs about fiemap. If offset unsigned, the result of
fiemap will leak some data.
Kernel commit eedf32bfcace7d8e20cc66757d74fc68f3439ff7 fix this bug.
To avoid this bug on kernel which don't apply this commit, in ceph we
make the offset down aligned with CEPH_PAGE_SIZE.
Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>