powerpc/syscalls: Simplify do_mmap2()

When shift is nul, operations remain valid so no test needed.

And 'ret' is unnecessary.

And use IS_ALIGNED() to check alignment, that's more clear.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/373ec500f386374bc5735007df3d3869eac47be1.1624618701.git.christophe.leroy@csgroup.eu