lightnvm: pblk: cleanly fail when there is not enough memory

lightnvm: pblk: cleanly fail when there is not enough memory

L2P table can be huge in many cases, since it typically requires 1GB
of DRAM for 1TB of drive. When there is not enough memory available,
OOM killer turns on and kills random processes, which can be very
annoying for users.

This patch changes the flag for L2P table allocation on order to handle
this situation in more user friendly way.

GFP_KERNEL and __GPF_HIGHMEM are default flags used in parameterless
vmalloc() calls, so they are also keeped in that patch. Additionally
__GFP_NOWARN flag is added in order to hide very long dmesg warn in
case of the allocation failures. The most important flag introduced
in that patch is __GFP_RETRY_MAYFAIL, which would cause allocator
to try use free memory and if not available to drop caches, but not
to run OOM killer.

Signed-off-by: Igor Konopko <igor.j.konopko@intel.com>
Reviewed-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Reviewed-by: Javier González <javier@javigon.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>