eal: add shared indexed file-backed array

eal: add shared indexed file-backed array

rte_fbarray is a simple indexed array stored in shared memory
via mapping files into memory. Rationale for its existence is the
following: since we are going to map memory page-by-page, there
could be quite a lot of memory segments to keep track of (for
smaller page sizes, page count can easily reach thousands). We
can't really make page lists truly dynamic and infinitely expandable,
because that involves reallocating memory (which is a big no-no in
multiprocess). What we can do instead is have a maximum capacity as
something really, really large, and decide at allocation time how
big the array is going to be. We map the entire file into memory,
which makes it possible to use fbarray as shared memory, provided
the structure itself is allocated in shared memory. Per-fbarray
locking is also used to avoid index data races (but not contents
data races - that is up to user application to synchronize).

In addition, in understanding that we will frequently need to scan
this array for free space and iterating over array linearly can
become slow, rte_fbarray provides facilities to index array's
usage. The following use cases are covered:
 - find next free/used slot (useful either for adding new elements
   to fbarray, or walking the list)
 - find starting index for next N free/used slots (useful for when
   we want to allocate chunk of VA-contiguous memory composed of
   several pages)
 - find how many contiguous free/used slots there are, starting
   from specified index (useful for when we want to figure out
   how many pages we have until next hole in allocated memory, to
   speed up some bulk operations where we would otherwise have to
   walk the array and add pages one by one)

This is accomplished by storing a usage mask in-memory, right
after the data section of the array, and using some bit-level
magic to figure out the info we need.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Tested-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Tested-by: Gowrishankar Muthukrishnan <gowrishankar.m@linux.vnet.ibm.com>