Fix up .gitignore for top-level file patterns

Some of the gitignore file patters were explicitly meant to be only for
the top level, but weren't marked that way, so they would trigger
recursively in subdirectories too. Normally that was harmless, but at
least "linux" happened to trigger elsewhere too. Fix it up.

And other patterns in that section weren't necessarily top-level at all.

Reported-by: Jiri Slaby <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

.gitignore

···
 #
 # Top-level generic files
 #
-tags
-TAGS
-linux
-vmlinux
-vmlinuz
-System.map
-Module.markers
-Module.symvers
+/tags
+/TAGS
+/linux
+/vmlinux
+/vmlinuz
+/System.map
+/Module.markers
+/Module.symvers
+
+#
+# git files that we don't want to ignore even it they are dot-files
+#
 !.gitignore
 !.mailmap