client: fix readdir + cap recovation/reissue race

client: fix readdir + cap recovation/reissue race

The current race:
 - we start readdir
 - mds revokes dir cap
 - some file gets removed
 - mds reissues dir cap
 - we finish readdir and set I_COMPLETE

We should only set it if there have been no FILE_SHARED reissues during
the readdir.

Note that we still set I_COMPLETE even if we don't have the cap; that's
useless but harmless, since it is undefined without FILE_SHARED being
set.

Signed-off-by: Sage Weil <sage@newdream.net>