Reorganize the handling all-zeroes terminal block in sparse mode

The intent of the previous code in that case was to force
an explicit write, but the implementation was incorrect, and
as a result the write was never performed. This new implementation
instead uses ftruncate(2) to extend the file with a trailing hole.

Also introduce regression tests for these cases.

PR: 189284
(original PR whose fix introduced this bug)

PR: 207092

Differential Revision: D5248
Reviewed by: sobomax,kib
MFC after: 2 weeks