Make use of kvmalloc if available and fix vmem_alloc implementation

Make use of kvmalloc if available and fix vmem_alloc implementation

This patch implements use of kvmalloc for GFP_KERNEL allocations, which
may increase performance if the allocator is able to allocate physical
memory, if kvmalloc is available as a public kernel interface (since
v4.12). Otherwise it will simply fall back to virtual memory (vmalloc).

Also fix vmem_alloc implementation which can lead to slow allocations
since the first attempt with kmalloc does not make use of the noretry
flag but tells the linux kernel to retry several times before it fails.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Ahrens <matt@delphix.com>
Signed-off-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Closes #9034