fib: Fix for existence check on Big Endian

fib: Fix for existence check on Big Endian

Adjust the expression size to 1B so cmp expression value is correct.
Without this, the rule 'fib saddr . iif check exists' generates
following byte code on BE:

|  [ fib saddr . iif oif present => reg 1 ]
|  [ cmp eq reg 1 0x00000001 ]

Though with NFTA_FIB_F_PRESENT flag set, nft_fib.ko writes to the first
byte of reg 1 only (using nft_reg_store8()). With this patch in place,
byte code is correct:

|  [ fib saddr . iif oif present => reg 1 ]
|  [ cmp eq reg 1 0x01000000 ]

Fixes: f686a17eafa0b ("fib: Support existence check")
Cc: Yi Chen <yiche@redhat.com>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>