(libT) better testing for fallocate() on linux

configure.ac

@@ -49,7 +49,14 @@ fi
 
 AC_HEADER_STDC
 AC_HEADER_TIME
-AC_CHECK_FUNCS([lrintf strlcpy daemon dirname basename daemon strcasecmp localtime_r fallocate posix_fallocate])
+
+AC_MSG_CHECKING([for fallocate])
+AC_TRY_LINK([#include <linux/falloc.h>],
+            [return fallocate(-1,0,0,0);],
+            [AC_DEFINE([HAVE_FALLOCATE],[1],[Defined if fallocate() exists])
+             AC_MSG_RESULT([yes])],
+            [AC_MSG_RESULT([no])])
+AC_CHECK_FUNCS([lrintf strlcpy daemon dirname basename daemon strcasecmp localtime_r posix_fallocate])
 AC_PROG_INSTALL
 AC_PROG_MAKE_SET
 ACX_PTHREAD

libtransmission/fdlimit.c

@@ -36,6 +36,10 @@
 #include <fcntl.h>
 #endif
 
+#ifdef HAVE_FALLOCATE
+ #include <linux/falloc.h>
+#endif
+
 #include <sys/types.h>
 #include <sys/stat.h>
 #ifdef HAVE_GETRLIMIT
@@ -104,7 +108,7 @@ preallocateFile( int fd UNUSED, uint64_t length UNUSED )
 {
 #ifdef HAVE_FALLOCATE
 
-    return fallocate( fd, 0, offset, length );
+    return fallocate( fd, FALLOC_FL_KEEP_SIZE, 0, length );
 
 #elif defined(HAVE_POSIX_FALLOCATE)