Bug 1640612 - Allow FIONREAD ioctl for socket process, r=jld

Differential Revision: https://phabricator.services.mozilla.com/D79071

security/sandbox/linux/SandboxFilter.cpp

@@ -1596,6 +1596,8 @@ class SocketProcessSandboxPolicy final : public SandboxPolicyCommon {
         return If(request == FIOCLEX, Allow())
             // Rust's stdlib also uses FIONBIO instead of equivalent fcntls.
             .ElseIf(request == FIONBIO, Allow())
+            // This is used by PR_Available in nsSocketInputStream::Available.
+            .ElseIf(request == FIONREAD, Allow())
             // ffmpeg, and anything else that calls isatty(), will be told
             // that nothing is a typewriter:
             .ElseIf(request == TCGETS, Error(ENOTTY))