Fix MemfdTest_OtherProcessCanOpenFromProcfs.
Fix MemfdTest_OtherProcessCanOpenFromProcfs.

- Make the body of InForkedProcess async-signal-safe.

- Pass the correct path to open().

PiperOrigin-RevId: 241348774
Change-Id: I753dfa36e4fb05521e659c173e3b7db0c7fc159b

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