slab allocator: fixes/enhancements in slab allocator

[dkgupta-amzn]

Fixes:
When we first implemented slab allocator for KTF, we never cared for freeing
memory when entire slab is freed up. Reason being we thought of it as short-
running kernel. However we're into a situation where we're allocating a lot of
memory, thanks for integration of external library doing malloc and free.
This change ensures that if a slab got completly freed up, we free up the memory
and put that meta slab back into free list.
Added a spinlock around allocation, free and initialization

Enhancement:
slab allocator was limited in the sense that it wouldn't allocate more if
meta_slab_t entries contained in globally allocated page at initializaiton
were all exhausted. This change makes a slight design change as follows:-

• During allocation if meta_slab_t is not available then allocator will do
  • Allocate a fresh page
  • First entry is special meta_slab_t in this page
  • First entry will treat rest of meta_slab_t entries in page as normal
    meta_slab_t entry to allocate user requested memory allocations.
  • Link special meta_slab_t entry in a global list so that all such allocated
    pages are linked together

Note about special meta_slab_t entry:-
Special meta_slab_t is basically to manage meta_slab_t entries itself. It's metadata
of metadata of user allocations.

Signed-off-by: dkgupta [email protected]