Recompute max variable after setting FD in the bitmap.

`fdBitmap.FirstZero()` could return `max` value; if it does, then
recompute the max value to avoid reusing the old max value twice.

The default bitmap size for file descriptors in gVisor is 65535.

Add a pipe test that attempts to create more than 65535 FDs to hit the edge
case where fdBitmap.FirstZero() returns the default bitmap max value of 65535.

TESTED:
http://sponge2/4c12ce75-3763-4773-ad62-87c6b8fe0446
http://sponge2/9c9d6ea0-b69c-432c-a16b-9446214109ba
PiperOrigin-RevId: 724410846

pkg/sentry/kernel/fd_table.go

@@ -267,6 +267,9 @@ func (f *FDTable) NewFDs(ctx context.Context, minFD int32, files []*vfs.FileDesc
 			break
 		}
 		f.fdBitmap.Add(fd)
+		if fd == uint32(max) {
+			max = int32(fd + 1)
+		}
 		if df := f.set(int32(fd), files[len(fds)], flags); df != nil {
 			panic("file set")
 		}

test/syscalls/linux/pipe.cc

@@ -691,6 +691,31 @@ TEST_P(PipeTest, ZeroSize) {
   ASSERT_THAT(read(rfd_.get(), nullptr, 0), SyscallSucceedsWithValue(0));
 }
 
+// Test that we can open more FDs than the max default value without crashing.
+TEST_P(PipeTest, PipeFdCount) {
+  SKIP_IF(!CreateBlocking());
+
+  // We make too many calls to go through full save cycles.
+  DisableSave ds;
+  constexpr size_t kMaxFd = 66000;
+  std::vector<int> fds;
+
+  while (true) {
+    int pipefd[2];
+    ASSERT_THAT(pipe2(pipefd, 0), SyscallSucceeds());
+    ASSERT_NE(pipefd[0], pipefd[1]);
+    fds.push_back(pipefd[0]);
+    fds.push_back(pipefd[1]);
+    if (static_cast<size_t>(pipefd[1]) > kMaxFd) {
+      break;
+    }
+  }
+
+  for (const auto fd : fds) {
+    close(fd);
+  }
+}
+
 std::string PipeCreatorName(::testing::TestParamInfo<PipeCreator> info) {
   return info.param.name_;  // Use the name specified.
 }