Fix clib_bitmap_next_clear() function when no clear bits left

Fix clib_bitmap_next_clear() function when no clear bits left

If the bitmap has no bit clear after the input bit position i,
the function will return i even if its bit is set.
Fix is to return the next bit just beyond the free bitmap.

This can cause IP neighbor scan crash in ip_neighbor_scan() with
a debug image. With production image, ip_neighbor_scan() may still
function, AFAICT, with extra neighbor delete attempts for entries
already deleted, until these entries are reused for new neighbors.

Change-Id: If6422ef6f63908ea39651de4ccbd8cb0b294bd69
Signed-off-by: John Lo <loj@cisco.com>