mds: check cap string only if !allow_all

mds: check cap string only if !allow_all

This corrects a regression introduced by #16891 which fixes
http://tracker.ceph.com/issues/20990. Not using cephx would
cause all clients to fail auth with:

    2017-08-17 12:21:05.191958 7f5b788d4700  0 -- 127.0.0.1:0/65887226 >> 127.0.0.1:6805/3339248996 conn(0x1004be8a0 :-1 s=STATE_CONNECTING_WAIT_CONNECT_REPLY_AUTH pgs=0 cs=0 l=0).handle_connect_reply connect got BADAUTHORIZER

Fixes: http://tracker.ceph.com/issues/21027
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>