XArray: Fix xas_create_range for ranges above 4 billion

The 'sibs' variable would be shifted as a 32-bit integer, so if 'shift'
is more than 32, this is undefined behaviour.  In practice, this doesn't
happen because the page cache is the only user and nobody uses 16TB pages.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>