ceph: use i_size_{read,write} to get/set i_size

ceph: use i_size_{read,write} to get/set i_size

Cap message from MDS can update i_size. In that case, we don't
hold i_mutex. So it's unsafe to directly access inode->i_size
while holding i_mutex.

Signed-off-by: Yan, Zheng <zyan@redhat.com>