libafs: Tidy up iovec allocation and trimming

libafs: Tidy up iovec allocation and trimming

Tidy up the way that we perform iovec allocation and trimming by
making the rest of the world look a little bit more like Darwin.

This relies upon a struct uio, followed by 16 iovecs, being able
to fit into a SmallSpace sized block. On the majority of 32 bit
systems, such a block is 256 bytes long (on AIX and HPUX it is
152 bytes). With a 32bit size_t, an iovec is 8 bytes, so 16 of
them is 128 bytes, and a struct uio is 24 bytes, giving a grand
total of 152.

Change-Id: I5243f034bcb4e1f5fa319c4f522229bd96eaae3d
Reviewed-on: http://gerrit.openafs.org/4749
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>