fix signedness of pgoff argument to remap_file_pages

both the kernel and glibc agree that this argument is unsigned; the
incorrect type ssize_t came from erroneous man pages.

include/sys/mman.h

@@ -33,7 +33,7 @@ int munlockall (void);
 
 #ifdef _GNU_SOURCE
 void *mremap (void *, size_t, size_t, int, ...);
-int remap_file_pages (void *, size_t, int, ssize_t, int);
+int remap_file_pages (void *, size_t, int, size_t, int);
 #endif
 
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)

src/linux/remap_file_pages.c

@@ -2,7 +2,7 @@
 #include <sys/mman.h>
 #include "syscall.h"
 
-int remap_file_pages(void *addr, size_t size, int prot, ssize_t pgoff, int flags)
+int remap_file_pages(void *addr, size_t size, int prot, size_t pgoff, int flags)
 {
 	return syscall(SYS_remap_file_pages, addr, size, prot, pgoff, flags);
 }