btrfs: add memalloc_nofs protections around alloc_workspace callback

The workspaces are preallocated at the beginning where we can safely use
GFP_KERNEL, but in some cases the find_workspace might reach the
allocation again, now in a more restricted context when the bios or
pages are being compressed.

To avoid potential lockup when alloc_workspace -> vmalloc would silently
use the GFP_KERNEL, add the memalloc_nofs helpers around the critical
call site.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>