Use the lnet_magic_accept() function to compare 'magic' against
LNET_PROTO_TCP_MAGIC for the appropriate string for an error message.
The original fix removed an unneeded byte-ordering cast because the define
was already CPU byte-ordered and it was assumed that 'magic' was CPU
byte-ordered, too.
Now modify the if-statement to use the appropriate lnet_accept_magic()
function in order to be consistent with similar tests. This will allow
the code to be consistent with the general understanding that 'magic'
should be in host-byte-order for the peer that sent the message.