fs/ntfs3: Fix integer overflow in multiplication

The multiplication of the u32 data_size with a int is being performed
using 32 bit arithmetic however the results is being assigned to the
variable nbits that is a size_t (64 bit) value. Fix a potential
integer overflow by casting the u32 value to a size_t before the
multiply to use a size_t sized bit multiply operation.

Addresses-Coverity: ("Unintentional integer overflow")
Fixes: 82cae269 ("fs/ntfs3: Add initialization of super block")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>