Ensure 64-bit off_t is used in user space instead of loff_t

Use 64-bit POSIX off_t in user space instead of the Linux kernel type
loff_t. This is enforced at configure time via AC_SYS_LARGEFILE and
AC_CHECK_SIZEOF([off_t]). loff_t remains in shared headers where they
mirror Linux VFS interfaces, and on FreeBSD we typedef loff_t to off_t
in those headers since libc does not provide it.

Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Moch <mail@alexmoch.com>
Closes #18020

lib/libspl/include/sys/kstat.h

@@ -86,6 +86,6 @@ extern void kstat_delete(kstat_t *);
 extern void kstat_set_raw_ops(kstat_t *ksp,
     int (*headers)(char *buf, size_t size),
     int (*data)(char *buf, size_t size, void *data),
-    void *(*addr)(kstat_t *ksp, loff_t index));
+    void *(*addr)(kstat_t *ksp, off_t index));
 
 #endif	/* _SYS_KSTAT_H */

lib/libspl/include/sys/types.h

@@ -50,19 +50,4 @@ typedef int		projid_t;
 
 #include <sys/param.h> /* for NBBY */
 
-#ifdef __FreeBSD__
-typedef off_t loff_t;
-#endif
-
-/*
- * On musl, loff_t is a macro within fcntl.h when _GNU_SOURCE is defined.
- * If no macro is defined, a typedef fallback is provided.
- */
-#if defined(__linux__) && !defined(__GLIBC__)
-#include <fcntl.h>
-#ifndef loff_t
-typedef off_t loff_t;
-#endif
-#endif
-
 #endif