Mark functions as static

Mark functions used only in the same translation unit as static. This
only includes functions that do not have a prototype in a header file
either.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10470

tests/zfs-tests/cmd/file_write/file_write.c

@@ -46,7 +46,7 @@ static void usage(char *);
 /*
  * psudo-randomize the buffer
  */
-void randomize_buffer(int block_size) {
+static void randomize_buffer(int block_size) {
 	int i;
 	char rnd = rand() & 0xff;
 	for (i = 0; i < block_size; i++)

tests/zfs-tests/cmd/libzfs_input_check/libzfs_input_check.c

@@ -918,7 +918,7 @@ enum zfs_ioc_ref {
  * Canonical reference check of /dev/zfs ioctl numbers.
  * These cannot change and new ioctl numbers must be appended.
  */
-boolean_t
+static boolean_t
 validate_ioc_values(void)
 {
 	boolean_t result = B_TRUE;

tests/zfs-tests/tests/functional/tmpfile/tmpfile_001_pos.c

@@ -28,7 +28,7 @@
 
 #define	BSZ 64
 
-void
+static void
 fill_random(char *buf, int len)
 {
 	int i;