Revert "kexec: Export kexec_in_progress to modules"

This reverts commit 97dcaa0f. Based on
the review discussion with Eric, we will come up with a different fix
for the bcm_sf2 driver which does not make it rely on the
kexec_in_progress value.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>