kexec-tools: avoid kernel warning

kexec-tools: avoid kernel warning

Running command 'kexec -p /path/to/crash-kernel --reuse-cmdline' will
give out the following warning:

  kexec[970]: memfd_create() called without MFD_EXEC or MFD_NOEXEC_SEAL set

Add MFD_NOEXEC_SEAL explicitly to avoid such warning.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>