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

tests/zfs-tests/cmd/clone_mmap_cached.c

@@ -36,12 +36,13 @@
 #include <string.h>
 #include <unistd.h>
 
-#ifdef __FreeBSD__
-#define	loff_t	off_t
+#if defined(_GNU_SOURCE) && defined(__linux__)
+_Static_assert(sizeof (loff_t) == sizeof (off_t),
+	"loff_t and off_t must be the same size");
 #endif
 
 ssize_t
-copy_file_range(int, loff_t *, int, loff_t *, size_t, unsigned int)
+copy_file_range(int, off_t *, int, off_t *, size_t, unsigned int)
     __attribute__((weak));
 
 static void *