bootconfig: Fix negative seeks on 32-bit with LFS enabled

Commit 26dda5769509 "tools/bootconfig: Cleanup bootconfig footer size
calculations" replaced some expressions of type int with the
BOOTCONFIG_FOOTER_SIZE macro, which expands to an expression of type
size_t, which is unsigned.

On 32-bit architectures with LFS enabled (i.e. off_t is 64-bit), the
seek offset of -BOOTCONFIG_FOOTER_SIZE now turns into a positive
value.

Fix this by casting the size to off_t before negating it.

Just in case someone changes BOOTCONFIG_MAGIC_LEN to have type size_t
later, do the same thing to the seek offset of -BOOTCONFIG_MAGIC_LEN.

Link: https://lore.kernel.org/all/[e-mail]/

Fixes: 26dda5769509 ("tools/bootconfig: Cleanup bootconfig footer size calculations")
Signed-off-by: Ben Hutchings <[e-mail]>
Signed-off-by: Masami Hiramatsu (Google) <[e-mail]>

tools/bootconfig/main.c

···
 	if (stat.st_size < BOOTCONFIG_FOOTER_SIZE)
 		return 0;
 
-	if (lseek(fd, -BOOTCONFIG_MAGIC_LEN, SEEK_END) < 0)
+	if (lseek(fd, -(off_t)BOOTCONFIG_MAGIC_LEN, SEEK_END) < 0)
 		return pr_errno("Failed to lseek for magic", -errno);
 
 	if (read(fd, magic, BOOTCONFIG_MAGIC_LEN) < 0)
···
 	if (memcmp(magic, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN) != 0)
 		return 0;
 
-	if (lseek(fd, -BOOTCONFIG_FOOTER_SIZE, SEEK_END) < 0)
+	if (lseek(fd, -(off_t)BOOTCONFIG_FOOTER_SIZE, SEEK_END) < 0)
 		return pr_errno("Failed to lseek for size", -errno);
 
 	if (read(fd, &size, sizeof(uint32_t)) < 0)