usdt: use ProcMountNSGuard for pid and path attach (#2064)
usdt: use ProcMountNSGuard for pid and path attach (#2064)

Since the probe may be in a descendent namespace, ensure that when `stat()`ing
for HINTs and resolving linked elf locations, be sure to always perform them
relative to any observed mount namespace.

However, usage of the mount namespace must be surgical, since we're observing
the pid with its outter identity, make sure we're not in the mount namespace
when attempting to `readlink` the `/proc/<pid>/exe`
brendangregg/bcc,iovisor/bcc,brendangregg/bcc,brendangregg/bcc,iovisor/bcc,brendangregg/bcc,iovisor/bcc,mcaleavya/bcc,mcaleavya/bcc,tuxology/bcc,brendangregg/bcc,tuxology/bcc,iovisor/bcc,tuxology/bcc,iovisor/bcc,tuxology/bcc,mcaleavya/bcc,mcaleavya/bcc,tuxology/bcc,mcaleavya/bcc