everywhere: misc unnecessary var init/update

These are all cases where we initialise or update a variable, and then
never use it. None of them particularly matter, as the compiler should
optimise them all away during dead store elimination, but some static
analysers complain about them and they are extra work for casual readers
to follow, so worth removing.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #17551

tests/zfs-tests/cmd/idmap_util.c

@@ -301,7 +301,7 @@ static int
 write_idmap(pid_t pid, char *buf, size_t buf_size, idmap_type_t type)
 {
 	char path[PATH_MAX];
-	int fd = -EBADF;
+	int fd;
 	int ret;
 
 	(void) snprintf(path, sizeof (path), "/proc/%d/%cid_map",

tests/zfs-tests/cmd/mktree.c

@@ -152,7 +152,7 @@ getfdname(char *pdir, char type, int level, int dir, int file)
 static void
 crtfile(char *pname)
 {
-	int fd = -1;
+	int fd;
 	int i, size;
 	const char *context = "0123456789ABCDF";
 	char *pbuf;

tests/zfs-tests/cmd/mmapwrite.c

@@ -59,7 +59,7 @@ static void *
 normal_writer(void *filename)
 {
 	char *file_path = filename;
-	int fd = -1;
+	int fd;
 	ssize_t write_num = 0;
 	int page_size = getpagesize();
 
@@ -93,7 +93,7 @@ normal_writer(void *filename)
 static void *
 map_writer(void *filename)
 {
-	int fd = -1;
+	int fd;
 	int ret = 0;
 	char *buf = NULL;
 	int page_size = getpagesize();