net: buf: Don't use recursion for freeing fragment list

Freeing the fragment list by recursively calling net_buf_unref() is
elegant but dangerous. Since we have fairly small stack sizes it'd be
possible to overflow the stack if the user creates a very long list of
fragments (empirical tests showed that some 21 fragments is enough to
kill a 2k stack). Instead, use a while-loop for freeing up the
fragments, thereby guaranteeing a fixed upper bound for net_buf_unref()
stack usage.

Change-Id: Ibfe794fa717d1cddc84365c7b7b9cff4024edbf6
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>