s3:vfs_default: use fsp_fnum_dbg() for fsp->fnum logging.

s3:vfs_default: use fsp_fnum_dbg() for fsp->fnum logging.

Note: it is actually wrong to access the fsp->fnum at all here,
since the fnum is part of the smb layer that should not be used
in the vfs layer. But this is subject be separated more cleanly
in later commits. This change only unifies the logging of fsp->fnum.

Signed-off-by: Stefan Metzmacher <metze@samba.org>