stat: increase the size of base to avoid overflow

stat: increase the size of base to avoid overflow

plat_idx_to_val() in stat.c has a variable base that is defined as an
unsigned int but it was designed to fit old settings where
FIO_IO_U_PLAT_GROUP_NR = 19.
With the alteration from usec to nsec in commit
d6bb626ef37d3905221ade2887b422717a07af09(nanosecond: update completion
latency recording and normal, json output to use nanoseconds)
FIO_IO_U_PLAT_GROUP_NR is now 29 and the range of latency changed from
uint32_t to uint64_t. This means in plat_idx_to_val(), variable base
can easily exceed a 32-bit range and overflow after error_bits reaches 26.

Eg.
Group	MSB	#discarded	range of		#buckets
		error_bits	value
27	32	26		[4294967296, 8589934591]     64
base = 1 << (error_bits + FIO_IO_U_PLAT_BITS);
(1 << 32) will overflow
if change to:
base = ((unsigned long long) 1) << (error_bits + FIO_IO_U_PLAT_BITS);
base itself will still overflow
so base should be unsigned long long