Protect PCIDs with a mutex.
Protect PCIDs with a mutex.

Because the Drop method may be called across vCPUs, it is necessary to protect
the PCID database with a mutex to prevent concurrent modification. The PCID is
assigned prior to entersyscall, so it's safe to block.

PiperOrigin-RevId: 207992864
Change-Id: I8b36d55106981f51e30dcf03e12886330bb79d67

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor