um: drivers: call kernel_strrchr() explicitly in cow_user.c

Building ARCH=um on glibc >= 2.43 fails:

  arch/um/drivers/cow_user.c: error: implicit declaration of
  function 'strrchr' [-Wimplicit-function-declaration]

glibc 2.43's C23 const-preserving strrchr() macro does not survive
UML's global -Dstrrchr=kernel_strrchr remap from arch/um/Makefile.
Call kernel_strrchr() directly in cow_user.c so the source no longer
depends on the -D rewrite.

Fixes: 2c51a4b ("um: fix strrchr() problems")
Suggested-by: Johannes Berg <[email protected]>
Cc: [email protected]
Assisted-by: Claude:claude-opus-4-6
Assisted-by: Codex:gpt-5-4
Signed-off-by: Michael Bommarito <[email protected]>
Link: https://patch.msgid.link/[email protected]
[remove unnecessary 'extern']
Signed-off-by: Johannes Berg <[email protected]>

Author: mjbommar

arch/um/drivers/cow_user.c

@@ -15,6 +15,12 @@
 #include "cow.h"
 #include "cow_sys.h"
 
+/*
+ * arch/um/Makefile remaps strrchr to kernel_strrchr; call the kernel
+ * name directly to avoid glibc >= 2.43's C23 strrchr macro.
+ */
+char *kernel_strrchr(const char *, int);
+
 #define PATH_LEN_V1 256
 
 /* unsigned time_t works until year 2106 */
@@ -153,7 +159,7 @@ static int absolutize(char *to, int size, char *from)
 			   errno);
 		return -1;
 	}
-	slash = strrchr(from, '/');
+	slash = kernel_strrchr(from, '/');
 	if (slash != NULL) {
 		*slash = '\0';
 		if (chdir(from)) {