rtnetlink: Replace implementation of ASSERT_RTNL() macro with WARN_ONCE()

ASSERT_RTNL() macro is actual open-coded variant of WARN_ONCE() with
two exceptions. First, it prints stack for multiple hits and not only
once as WARN_ONCE() does. Second, the user can disable prints of
WARN_ONCE by setting CONFIG_BUG to N.

The multiple prints of dump stack are actually not needed, because calls
without rtnl lock are programming errors and user can't do anything
about them except to complain to the mailing list after first occurrence
of such failure.

The user who disabled BUG/WARN prints did it explicitly because by default
in upstream kernel and distributions this option is enabled. It means
that user doesn't want to see prints about missing locks too.

This patch replaces open-coded variant in favor of already existing
macro and change error prints to be once only.

Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>