ceph: stop copying to iter at EOF on sync reads

If EOF is encountered, ceph_sync_read() return value is adjusted down
according to i_size, but the "to" iter is advanced by the actual number
of bytes read.  Then, when retrying, the remainder of the range may be
skipped incorrectly.

Ensure that the "to" iter is advanced only until EOF.

[ idryomov: changelog ]

Fixes: c3d8e0b5 ("ceph: return the real size read when it hits EOF")
Reported-by: Frank Hsiao <frankhsiao@qnap.com>
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Tested-by: Frank Hsiao <frankhsiao@qnap.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>