fastboot: Fix "fastboot gsi".

This command erroneously reports that no GSI is installed, because
/metadata is not mounted in recovery. To address this, temporarily mount
/metadata when the gsi command is invoked.

Bug: 122556707
Test: fastboot gsi disable
      fastboot gsi wipe

Change-Id: Ib21971b49b46fd580b902ff75f01cfb96192afc0
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index 17cab3a..4cdd8bc 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -1948,11 +1948,10 @@
             std::string size = next_arg(&args);
             fb->ResizePartition(partition, size);
         } else if (command == "gsi") {
-            if (args.empty()) {
-                syntax_error("missing 'wipe' or 'disable' argument");
-            } else if (args.size() == 1 && args[0] == "wipe") {
+            std::string arg = next_arg(&args);
+            if (arg == "wipe") {
                 fb->RawCommand("gsi:wipe", "wiping GSI");
-            } else if (args.size() == 1 && args[0] == "disable") {
+            } else if (arg == "disable") {
                 fb->RawCommand("gsi:disable", "disabling GSI");
             } else {
                 syntax_error("expected 'wipe' or 'disable'");