Move TDB_USERWR check under 'if (traced)'.

If we hadn't been traced in the first place when syscallenter()
started executing, we can ignore TDB_USERWR. TDB_USERWR can get set,
sure, but if it does, it's because the debugger raced with the syscall,
and it cannot depend on winning that race.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: EPSRC
Differential Revision: https://reviews.freebsd.org/D26585