staging: lustre: use kmalloc for allocating ksock_tx

The size of the data structure is primarily controlled
by the iovec size, which is limited to 256.
Entries in this vector are 12 bytes, so the whole
will always fit in a page.
So it is safe to use kmalloc (kvmalloc not needed).
So replace LIBCFS_ALLOC with kmalloc.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>