[XFS] Fix xfs_off_t to be signed, not unsigned; valid warnings emitted

after stricter compilation options used by some OSDL folks.

SGI Modid: xfs-linux:xfs-kern:174814a
Signed-off-by: Nathan Scott <nathans@sgi.com>

fs/xfs/xfs_types.h

@@ -58,7 +58,7 @@ typedef enum { B_FALSE,B_TRUE }	boolean_t;
 typedef __int64_t		prid_t;		/* project ID */
 typedef __uint32_t		inst_t;		/* an instruction */
 
-typedef __u64			xfs_off_t;
+typedef __s64			xfs_off_t;	/* <file offset> type */
 typedef __u64			xfs_ino_t;	/* <inode> type */
 typedef __s64			xfs_daddr_t;	/* <disk address> type */
 typedef char *			xfs_caddr_t;	/* <core address> type */