fs_types: file_layout_t: convert pool -1 (undefined) to 0 in legacy encoding

fs_types: file_layout_t: convert pool -1 (undefined) to 0 in legacy encoding

Old code assumes that fl_pg_pool == 0 means the pool is not defined, while
file_layout_t uses -1.  Translate between the two.

Note that this means a valid file_layout_t with pool_id == 0 cannot be
accurately translated to a legacy file_layout_t.  That is somewhat
unavoidable, and should not be a problem since real clusters create 'rbd'
as pool 0 and it does not use any file layouts.

Signed-off-by: Sage Weil <sage@redhat.com>