x86/mem_sharing: resolve mm-lock order violations when forking VMs with nested p2m

x86/mem_sharing: resolve mm-lock order violations when forking VMs with nested p2m

Several lock-order violations have been encountered while attempting to fork
VMs with nestedhvm=1 set. This patch resolves the issues.

The order violations stems from a call to p2m_flush_nestedp2m being performed
whenever the hostp2m changes. This functions always takes the p2m lock for the
nested_p2m. However, with sharing the p2m locks always have to be taken before
the sharing lock. To resolve this issue we avoid taking the sharing lock where
possible (and was actually unecessary to begin with). But we also make
p2m_flush_nestedp2m aware that the p2m lock may have already been taken and
preemptively take all nested_p2m locks before unsharing a page where taking the
sharing lock is necessary.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@intel.com>
Acked-by: Jan Beulich <jbeulich@suse.com>