ppc: Convert mmu context allocation to new IDA API

ida_alloc_range is the perfect fit for this use case.  Eliminates
a custom spinlock, a call to ida_pre_get and a local check for the
allocated ID exceeding a maximum.

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>