linux(4): Use l_off_t type for offset argument in sendfile syscall

The off_t on Linux is a long, so it's non-functional change, just to
avoid confusing future readers.

MFC after:		1 month

sys/amd64/linux32/syscalls.master

@@ -1020,7 +1020,7 @@
 		int linux_sendfile(
 		    l_int out,
 		    l_int in,
-		    l_long *offset,
+		    l_off_t *offset,
 		    l_size_t count
 		);
 	}

sys/i386/linux/syscalls.master

@@ -1038,7 +1038,7 @@
 		int linux_sendfile(
 		    l_int out,
 		    l_int in,
-		    l_long *offset,
+		    l_off_t *offset,
 		    l_size_t count
 		);
 	}