eal: fix signed integers in fbarray

eal: fix signed integers in fbarray

While debugging startup issues encountered with Clang (see "eal: fix
undefined behavior in fbarray"), I noticed that fbarray stores indices,
sizes and masks on signed integers involved in bitwise operations.

Such operations almost invariably cause undefined behavior with values that
cannot be represented by the result type, as is often the case with
bit-masks and left-shifts.

This patch replaces them with unsigned integers as a safety measure and
promotes a few internal variables to larger types for consistency.

Coverity issue: 272598, 272599
Fixes: c44d09811b40 ("eal: add shared indexed file-backed array")

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>