lib: add gen_pool_alloc_aligned()

This commit adds a gen_pool_alloc_aligned() function to the
generic allocator API.  It allows specifying alignment for the
allocated block.  This feature uses
the bitmap_find_next_zero_area_off() function.

It also fixes possible issue with bitmap's last element being
not fully allocated (ie. space allocated for chunk->bits is
not a multiple of sizeof(long)).

It also makes some other smaller changes:
- moves structure definitions out of the header file,
- adds __must_check to functions returning value,
- makes gen_pool_add() return -ENOMEM rater than -1 on error,
- changes list_for_each to list_for_each_entry, and
- makes use of bitmap_clear().

Change-Id: Ib59f54d51ffe4c4fb747defc97a6581c46565890
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>