[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.  It also cleans up some error handling.

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

tests/zfs-tests/cmd/mmapwrite.c

@@ -65,6 +65,7 @@ normal_writer(void *filename)
 	fd = open(file_path, O_RDWR | O_CREAT, 0777);
 	if (fd == -1) {
 		err(1, "failed to open %s", file_path);
+		return (NULL);
 	}
 
 	char buf = 'z';
@@ -82,6 +83,10 @@ normal_writer(void *filename)
 			break;
 		}
 	}
+
+	if (fclose(fd) != 0)
+		err(1, "failed to close file");
+
 	return (NULL);
 }