qcow2: Make qcow2_cache_table_release() work only in Linux

qcow2: Make qcow2_cache_table_release() work only in Linux

We are using QEMU_MADV_DONTNEED to discard the memory of individual L2
cache tables. The problem with this is that those semantics are
specific to the Linux madvise() system call. Other implementations of
madvise() (including the very Linux implementation of posix_madvise())
don't do that, so we cannot use them for the same purpose.

This patch makes the code Linux-specific and uses madvise() directly
since there's no point in going through qemu_madvise() for this.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>