[TIPC]: Enhancements to msg_set_bits() routine

This patch makes two enhancements to msg_set_bits():

1) It now ignores any bits of the new field value that are not
   covered by the mask being used.  (Previously, if the new value
   exceeded the size of the mask the extra bits could corrupt
   other fields in the message header word being updated.)

2) The code has been optimized to minimize the number of run-time
   endianness conversion operations by leveraging the fact that the
   mask (and, in some cases, the value as well) is constant and the
   necessary conversion can be performed by the compiler.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Jon Paul Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>