pmap: Degrade pmap_page_set_attr*() into a no-op on same attribute

[olce]

Summary

For 32-bit arm, move the no-op test that was already in place at start
of the function so that it stays first even if the '#if 0' block around
the call to sf_buf_invalidate_cache() is uncommented at some point (if
ever).

[jeffpc_josefsipek.net]

As a non-expert on the code, LGTM (assuming there aren't some special rules about read-write accesses to the saved attributes).

sys/powerpc/aim/mmu_radix.c

5942

Trivial nit: other files got an extra empty line after the if-return.