btf: some inflateRawTypes simplifications

[lmb]

btf: remove fixupAndAssert

fixupAndAssert in inflateRawTypes is used to assert that a Type value has a
certain concrete type. The only use is validating that Func.Type is a
FuncProto so that a later check of Func.Type can type assert without error
checking. This is a bad idea since understanding why the assertion is safe 
now requires understanding how types are inflated.

Remove fixupAndAssert and instead check for errors during the type
assertion.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

btf: rename maxTypeDepth to maxResolveDepth

Rename the constant and document where it originates. The previous name is
misleading since vmlinux BTF does contain types which are nested more deeply
than 32 items at the moment.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>