checkpatch: add error on use of attribute((weak)) or __weak declarations

Using weak declarations can have unintended link defects.  The __weak on
the declaration causes non-weak definitions to become weak.

Emit an error on its use.
Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -4671,6 +4671,15 @@ sub process {
 			}
 		}
 
+# Check for __attribute__ weak, or __weak declarations (may have link issues)
+		if ($^V && $^V ge 5.10.0 &&
+		    $line =~ /(?:$Declare|$DeclareMisordered)\s*$Ident\s*$balanced_parens\s*(?:$Attribute)?\s*;/ &&
+		    ($line =~ /\b__attribute__\s*\(\s*\(.*\bweak\b/ ||
+		     $line =~ /\b__weak\b/)) {
+			ERROR("WEAK_DECLARATION",
+			      "Using weak declarations can have unintended link defects\n" . $herecurr);
+		}
+
 # check for sizeof(&)
 		if ($line =~ /\bsizeof\s*\(\s*\&/) {
 			WARN("SIZEOF_ADDRESS",