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

cmd/zinject/zinject.c

@@ -613,7 +613,7 @@ register_handler(const char *pool, int flags, zinject_record_t *record,
 	return (0);
 }
 
-int
+static int
 perform_action(const char *pool, zinject_record_t *record, int cmd)
 {
 	zfs_cmd_t zc = {"\0"};