qa: skip chdir for fuse_mount

qa: skip chdir for fuse_mount

The use of chdir will muck up the use of nsenter with valgrind:

    2021-03-03T02:13:49.897 DEBUG:teuthology.orchestra.run.smithi144:> sudo nsenter --net=/var/run/netns/ceph-ns--home-ubuntu-cephtest-mnt.0 cd /home/ubuntu/cephtest && sudo adjust-ulimits ceph-coverage /home/ubuntu/cephtest/archive/coverage daemon-helper term env 'OPENSSL_ia32cap=~0x1000000000000000' valgrind --trace-children=no --child-silent-after-fork=yes '--soname-synonyms=somalloc=*tcmalloc*' --num-callers=50 --suppressions=/home/ubuntu/cephtest/valgrind.supp --xml=yes --xml-file=/var/log/ceph/valgrind/client.0.log --time-stamp=yes --vgdb=yes --exit-on-first-error=yes --error-exitcode=42 --tool=memcheck --leak-check=full --show-reachable=yes ceph-fuse -f --admin-socket '/var/run/ceph/$cluster-$name.$pid.asok' --id 0 /home/ubuntu/cephtest/mnt.0
    2021-03-03T02:13:49.899 DEBUG:teuthology.orchestra.run.smithi144:> sudo modprobe fuse
    2021-03-03T02:13:49.914 INFO:teuthology.orchestra.run:Running command with timeout 30
    2021-03-03T02:13:49.914 DEBUG:teuthology.orchestra.run.smithi144:> sudo mount -t fusectl /sys/fs/fuse/connections /sys/fs/fuse/connections
    2021-03-03T02:13:49.919 INFO:tasks.cephfs.fuse_mount.ceph-fuse.0.smithi144.stderr:nsenter: failed to execute cd: No such file or directory

It's not necessary to chdir at all to do the mount, so don't.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>