blist_meta_alloc assumes that mask=scan->bm_bitmap is nonzero. But if the
cursor lies in the middle of the space that the meta node represents, then
blanking the low bits of mask may make it zero, and break later code that
expects a nonzero value. Add a test that returns failure if the mask has
been cleared.

Submitted by: Doug Moore <dougm@rice.edu>
Reported by: pho
Tested by: pho
X-MFC with: r340402
Differential Revision: https://reviews.freebsd.org/D18058