hugetlb: fix size=4G parsing

On 32-bit machines, mount -t hugetlbfs -o size=4G gave a 0GB filesystem,
size=5G gave a 1GB filesystem etc: there's no point in masking size with
HPAGE_MASK just before shifting its lower bits away, and since HPAGE_MASK is a
UL, that removed all the higher bits of the unsigned long long size.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>