unionfs: hold mount interlock while manipulating mnt_flag

This is for consistency with other filesystems.
Provide vfs suspension support only for filesystems which need it, take
two.

nullfs and unionfs need to request suspension if underlying filesystem(s)
use it. Utilize mnt_kern_flag for this purpose.

This is a fixup for 273271.

No strong objections from: kib
Pointy hat to: mjg
MFC after:	2 weeks
Fix the mtx_sleep() error checking, catch all errors and not only
EWOULDBLOCK.

Do not print any message at errors.  The errors are properly sent to upper
layers which should be able to deal with it, including printing the errors
when they need to.

The error message was quite annoying while scanning the i2c bus.

MFC after:	1 week
Merge from projects/bhyve_svm all the changes outside vmm.ko or bhyve utilities:

Add support for AMD's nested page tables in pmap.c:
- Provide the correct bit mask for various bit fields in a PTE (e.g. valid bit)
  for a pmap of type PT_RVI.
- Add a function 'pmap_type_guest(pmap)' that returns TRUE if the pmap is of
  type PT_EPT or PT_RVI.

Add CPU_SET_ATOMIC_ACQ(num, cpuset):
This is used when activating a vcpu in the nested pmap. Using the 'acquire'
variant guarantees that the load of the 'pm_eptgen' will happen only after
the vcpu is activated in 'pm_active'.

Add defines for various AMD-specific MSRs.

Submitted by:	Anish Gupta (akgupt3@gmail.com)