net: bpfilter: change section name of bpfilter UMH blob.

The section of bpfilter UMH blob is the ".bpfilter_umh". but this is not
an explicit section. so linking warning occurred at compile time for the
powerpc.
So, this patch makes use of the ".rodata" instead of the ".bpfilter_umh".

Config condition:

CONFIG_BPFILTER=y
CONFIG_BPFILTER_UMH=y

Result:

ld: warning: orphan section `.bpfilter_umh' from
`net/bpfilter/bpfilter_umh_blob.o' being placed in section `.bpfilter_umh'

Fixes: 61fbf593

 ("net: bpfilter: restart bpfilter_umh when error occurred")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>