Move check_file to os/$platform section

Keep check_file_generic() in shared code base, and allow special case
code in check_file() in os section. In future, macOS will have
additional checks in check_file().

Linux and FreeBSD wrappers just calls check_file_generic().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #12385

cmd/zpool/zpool_vdev.c

@@ -112,7 +112,7 @@ vdev_error(const char *fmt, ...)
  * not in use by another pool, and not in use by swap.
  */
 int
-check_file(const char *file, boolean_t force, boolean_t isspare)
+check_file_generic(const char *file, boolean_t force, boolean_t isspare)
 {
 	char  *name;
 	int fd;