initramfs: allocate heap buffers together

header_buf, symlink_buf and name_buf all share the same lifecycle so
needn't be allocated / freed separately. This change leads to a minor
reduction in .text size:

before:
   text    data     bss     dec     hex filename
   7914    1110       8    9032    2348 init/initramfs.o

after:
   text    data     bss     dec     hex filename
   7854    1110       8    8972    230c init/initramfs.o

A previous iteration of this patch reused a single buffer instead of
three, given that buffer use is state-sequential (GotHeader, GotName,
GotSymlink). However, the slight decrease in heap use during early boot
isn't really worth the extra review complexity.

Link: https://lore.kernel.org/all/20241107002044.16477-7-ddiss@suse.de/
Signed-off-by: David Disseldorp <ddiss@suse.de>
Link: https://lore.kernel.org/r/20250304061020.9815-6-ddiss@suse.de
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: ddiss

init/initramfs.c

@@ -510,14 +510,19 @@ char * __init unpack_to_rootfs(char *buf, unsigned long len)
 	decompress_fn decompress;
 	const char *compress_name;
 	static __initdata char msg_buf[64];
+	struct {
+		char header[CPIO_HDRLEN];
+		char symlink[PATH_MAX + N_ALIGN(PATH_MAX) + 1];
+		char name[N_ALIGN(PATH_MAX)];
+	} *bufs = kmalloc(sizeof(*bufs), GFP_KERNEL);
 
-	header_buf = kmalloc(CPIO_HDRLEN, GFP_KERNEL);
-	symlink_buf = kmalloc(PATH_MAX + N_ALIGN(PATH_MAX) + 1, GFP_KERNEL);
-	name_buf = kmalloc(N_ALIGN(PATH_MAX), GFP_KERNEL);
-
-	if (!header_buf || !symlink_buf || !name_buf)
+	if (!bufs)
 		panic_show_mem("can't allocate buffers");
 
+	header_buf = bufs->header;
+	symlink_buf = bufs->symlink;
+	name_buf = bufs->name;
+
 	state = Start;
 	this_header = 0;
 	message = NULL;
@@ -560,9 +565,7 @@ char * __init unpack_to_rootfs(char *buf, unsigned long len)
 		len -= my_inptr;
 	}
 	dir_utime();
-	kfree(name_buf);
-	kfree(symlink_buf);
-	kfree(header_buf);
+	kfree(bufs);
 	return message;
 }