mds: do not shift to EXCL or MIX while rdlocked

mds: do not shift to EXCL or MIX while rdlocked

There was an old change in file_eval() that was allowing us to switch from
SYNC to MIX or EXCL while there were rdlocks, which either caused lots of
lock thrashing or could (I think) hang things up completely.  This was
from ea10a672, an ancient fix for something related that appears to have
taken out the rdlocked check by accident.

In my tests (one writer, one stat-er), this took things from long stalls
(up to 20 seconds) to very responsive stats.  Yay!

Fixes: #791
Signed-off-by: Sage Weil <sage.weil@dreamhost.com>