client: fix "ceph.snap.btime" vxattr value

The vxattr value incorrectly places a "09" prefix to the nanoseconds
field, instead of providing it as a zero-pad width specifier after '%'.

Fixes: https://tracker.ceph.com/issues/39705
Signed-off-by: David Disseldorp <ddiss@suse.de>

src/client/Client.cc

@@ -11808,7 +11808,7 @@ bool Client::_vxattrcb_snap_btime_exists(Inode *in)
 
 size_t Client::_vxattrcb_snap_btime(Inode *in, char *val, size_t size)
 {
-  return snprintf(val, size, "%llu.09%lu",
+  return snprintf(val, size, "%llu.%09lu",
       (long long unsigned)in->snap_btime.sec(),
       (long unsigned)in->snap_btime.nsec());
 }