mds: make strncpy in ::respawn safer

Previous code assumed null terminated argv[0]
was not longer than PATH_MAX and the resulting
strncpy was not strictly safe.

Modify the bounds to ensure that copy will not
result in an unterminated string if argv[0]
is oversized.

Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #1711 from ceph/wip-coverity-respawn

mds: make strncpy in ::respawn safer

Reviewed-by: Sage Weil <sage@inktank.com>