kexec: fix Oops in crash_shrink_memory()

kexec: fix Oops in crash_shrink_memory()

When crashkernel is not enabled, "echo 0 > /sys/kernel/kexec_crash_size"
OOPSes the kernel in crash_shrink_memory.  This happens when
crash_shrink_memory tries to release the 'crashk_res' resource which are
not reserved.  Also value of "/sys/kernel/kexec_crash_size" shows as 1,
which should be 0.

This patch fixes the OOPS in crash_shrink_memory and shows
"/sys/kernel/kexec_crash_size" as 0 when crash kernel memory is not
reserved.

Signed-off-by: Pavan Naregundi <[email protected]>
Reviewed-by: WANG Cong <[email protected]>
Cc: Simon Horman <[email protected]>
Cc: Vivek Goyal <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>