mds: Locker: fix a NULL deref in _update_cap_fields

The MClientCaps* is allowed to be NULL, so we can't deref it unless
the dirty param is non-zero. So don't do the ahead-of-time lookup;
just call it explicitly in the if block.

Signed-off-by: Greg Farnum <greg@inktank.com>
Merge pull request #2679 from ceph/giant-locker-null

mds: Locker: fix a NULL deref in _update_cap_fields