Move the implementation of tdp_mmu_alloc_sp_for_split() to its one and
only caller to reduce unnecessary nesting and make it more clear why the
eager split loop continues after allocating a new SP.

Opportunistically drop the double-underscores from
__tdp_mmu_alloc_sp_for_split() now that its parent is gone.

No functional change intended.
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20240611220512.2426439-4-dmatlack@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>