[ZTS] fclose 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.  ee6bf97 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 fclose()-ing the file at
the end of the thread.

Signed-off-by: Antonio Russo <aerusso@aerusso.net>

tests/zfs-tests/cmd/mmapwrite.c

@@ -82,6 +82,10 @@ normal_writer(void *filename)
 			break;
 		}
 	}
+
+	if (close(fd) != 0)
+		err(1, "failed to close file");
+
 	return (NULL);
 }