ceph: initialize i_size variable in ceph_sync_read

Newer compilers seem to determine that this variable being uninitialized
isn't a problem, but older compilers (from the RHEL8 era) seem to choke
on it and complain that it could be used uninitialized.

Go ahead and initialize the variable at declaration time to silence
potential compiler warnings.

Fixes: c3d8e0b5 ("ceph: return the real size read when it hits EOF")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>