ceph: don't update importing cap's mseq when handing cap export

Updating mseq makes client think importer mds has accepted all prior
cap messages and importer mds knows what caps client wants. Actually
some cap messages may have been dropped because of mseq mismatch.

If mseq is left untouched, importing cap's mds_wanted later will get
reset by cap import message.

Cc: stable@vger.kernel.org
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>