net: Let's use inline function for type checking for net_buf

Using macros does not let the compiler verifying about the type we are
providing, which usually give an error easier to understand.
Also, this will let the compiler deciding how to actually optimize
(inline or not) the code.

Change-Id: I17fb1f5a1c1854461fad101bbb40c9be33844c8b
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>