ovl: fix typo in MODULE_PARM_DESC

ovl: fix typo in MODULE_PARM_DESC

Change first argument to MODULE_PARM_DESC() calls, that each of them
matched the actual module parameter name.  The matching results in
changing (the 'parm' section from) the output of `modinfo overlay` from:

    parm: ovl_check_copy_up:Obsolete; does nothing
    parm: redirect_max:ushort
    parm: ovl_redirect_max:Maximum length of absolute redirect xattr value
    parm: redirect_dir:bool
    parm: ovl_redirect_dir_def:Default to on or off for the redirect_dir feature
    parm: redirect_always_follow:bool
    parm: ovl_redirect_always_follow:Follow redirects even if redirect_dir feature is turned off
    parm: index:bool
    parm: ovl_index_def:Default to on or off for the inodes index feature
    parm: nfs_export:bool
    parm: ovl_nfs_export_def:Default to on or off for the NFS export feature
    parm: xino_auto:bool
    parm: ovl_xino_auto_def:Auto enable xino feature
    parm: metacopy:bool
    parm: ovl_metacopy_def:Default to on or off for the metadata only copy up feature

into:

    parm: check_copy_up:Obsolete; does nothing
    parm: redirect_max:Maximum length of absolute redirect xattr value (ushort)
    parm: redirect_dir:Default to on or off for the redirect_dir feature (bool)
    parm: redirect_always_follow:Follow redirects even if redirect_dir feature is turned off (bool)
    parm: index:Default to on or off for the inodes index feature (bool)
    parm: nfs_export:Default to on or off for the NFS export feature (bool)
    parm: xino_auto:Auto enable xino feature (bool)
    parm: metacopy:Default to on or off for the metadata only copy up feature (bool)

Signed-off-by: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>