mds: verify backtrace when fetching dirfrag

Fixes: #9557
Signed-off-by: Yan, Zheng <zyan@redhat.com>
This name doesn't work — we're always checking the backtrace. This value just determines if we crash on a bad backtrace.
I think it's probably okay if we unconditionally check backtraces, so let's change this name to "mds_require_valid_backtrace".