eal: fix undefined behavior in fbarray

eal: fix undefined behavior in fbarray

According to GCC documentation [1], the __builtin_clz() family of functions
yield undefined behavior when fed a zero value. There is one instance in
the fbarray code where this can occur.

Clang (at least version 3.8.0-2ubuntu4) seems much more sensitive to this
than GCC and yields random results when compiling optimized code, as shown
below:

 #include <stdio.h>

 int main(void)
 {
         volatile unsigned long long moo;
         int x;

         moo = 0;
         x = __builtin_clzll(moo);
         printf("%d\n", x);
         return 0;
 }

 $ gcc -O3 -o test test.c && ./test
 63
 $ clang -O3 -o test test.c && ./test
 1742715559
 $ clang -O0 -o test test.c && ./test
 63

Even 63 can be considered an unexpected result given the number of leading
zeroes should be the full width of the underlying type, i.e. 64.

In practice it causes find_next_n() to sometimes return negative values
interpreted as errors by caller functions, which prevents DPDK applications
from starting due to inability to find free memory segments:

 # testpmd [...]
 EAL: Detected 32 lcore(s)
 EAL: Detected 2 NUMA nodes
 EAL: No free hugepages reported in hugepages-1048576kB
 EAL: Multi-process socket /var/run/.rte_unix
 EAL: eal_memalloc_alloc_seg_bulk(): couldn't find suitable memseg_list
 EAL: FATAL: Cannot init memory

 EAL: Cannot init memory

 PANIC in main():
 Cannot init EAL
 4: [./build/app/testpmd(_start+0x29) [0x462289]]
 3: [/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)
     [0x7f19d54fc830]]
 2: [./build/app/testpmd(main+0x8a3) [0x466193]]
 1: [./build/app/testpmd(__rte_panic+0xd6) [0x4efaa6]]
 Aborted

This problem appears with commit 66cc45e293ed ("mem: replace memseg with
memseg lists") however the root cause is introduced by a prior patch.

[1] https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

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>