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

Commit 26dda576 "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/aKHlevxeg6Y7UQrz@decadent.org.uk/



Fixes: 26dda576 ("tools/bootconfig: Cleanup bootconfig footer size calculations")
Signed-off-by: Ben Hutchings <benh@debian.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>