UBIFS: fix compiler warnings

fs/ubifs/super.c: In function ‘ubifs_show_options’:
fs/ubifs/super.c:425: warning: format not a string literal and no format arguments
fs/ubifs/super.c: In function ‘mount_ubifs’:
fs/ubifs/super.c:1204: warning: format not a string literal and no format arguments
fs/ubifs/super.c: In function ‘ubifs_remount_rw’:
fs/ubifs/super.c:1557: warning: format not a string literal and no format arguments

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>