fuse: add support for SEEK_HOLE and SEEK_DATA in lseek

A useful performance improvement for accessing virtual machine images
via FUSE mount.

See https://bugzilla.redhat.com/show_bug.cgi?id=1220173 for a use-case
for glusterFS.

Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>

fs/fuse/fuse_i.h

@@ -605,6 +605,9 @@ struct fuse_conn {
 	/** Does the filesystem support asynchronous direct-IO submission? */
 	unsigned async_dio:1;
 
+	/** Is lseek not implemented by fs? */
+	unsigned no_lseek:1;
+
 	/** The number of requests waiting for completion */
 	atomic_t num_waiting;