arm64: kexec_file: fix memory leakage in create_dtb() when fdt_open_into() fails

in function create_dtb(), if fdt_open_into() fails, we need to vfree
buf before return.

Fixes: 52b2a8af ("arm64: kexec_file: load initrd and device-tree")
Cc: stable@vger.kernel.org # v5.0
Signed-off-by: qiuguorui1 <qiuguorui1@huawei.com>
Link: https://lore.kernel.org/r/20210218125900.6810-1-qiuguorui1@huawei.com


Signed-off-by: Will Deacon <will@kernel.org>