tools/bootconfig: fix fd leak in load_xbc_file() on fstat failure

tools/bootconfig: fix fd leak in load_xbc_file() on fstat failure

[ Upstream commit 3b2c2ab4ceb82af484310c3087541eab00ea288b ]

If fstat() fails after open() succeeds, the function returns without
closing the file descriptor. Also preserve errno across close(), since
close() may overwrite it before the error is returned.

Link: https://lore.kernel.org/all/20260318155847.78065-3-objecting@objecting.org/
Fixes: 950313ebf79c ("tools: bootconfig: Add bootconfig command")
Signed-off-by: Josh Law <objecting@objecting.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>