ZTS: close in mmapwrite.c

mmapwrite is used during the ZTS to identify issues with mmap-ed files.
This helper program exercises this pathway by continuously writing to a
file. ee6bf97c7 modified the writing threads to terminate after a set
amount of total data is written. This change allows standard program
execution to reach the end of a writer thread without closing the file
descriptor, introducing a resource "leak."

This patch appeases resource leak analyses by close()-ing the file at
the end of the thread.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Antonio Russo <aerusso@aerusso.net>
Closes #14353