net: ipa: separate memory validation from initialization

Currently, memory regions are validated in the loop that initializes
them.  Instead, validate them separately.

Rename ipa_mem_valid() to be ipa_mem_valid_one().  Define a *new*
function named ipa_mem_valid() that performs validation of the array
of memory regions provided.  This function calls ipa_mem_valid_one()
for each region in turn.

Skip validation for any "empty" region descriptors, which have zero
size and are not preceded by any canary values.  Issue a warning for
such descriptors if the offset is non-zero.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>