UBIFS: fix integer overflow warning

Fix the following warning:

fs/ubifs/io.c: In function 'ubifs_wbuf_init':
fs/ubifs/io.c:860: warning: integer overflow in expression

And limit maximum hrtimer delta to ULONG_MAX because the
argument is 'unsigned long'.

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