Both vn_close() and VFS_PROLOGUE() evaluate vp->v_mount twice, without
holding the vnode lock; vp->v_mount is checked first for NULL
equiality, and then dereferenced if not NULL.  If vnode is reclaimed
meantime, second dereference would still give NULL.  Change
VFS_PROLOGUE() to evaluate the mp once, convert MNTK_SHARED_WRITES and
MNTK_EXTENDED_SHARED tests into inline functions.

Reviewed by:	alc
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
If filesystem declares that it supports shared locking for writes, use
shared vnode lock for VOP_PUTPAGES() as well.  The only such
filesystem in the tree is ZFS, and it uses
vnode_pager_generic_putpages(), which performs the pageout with
VOP_WRITE().

Reviewed by:	alc
Discussed with:	avg
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Fix (erroneous) gcc warnings about usage of uninitialized variables in
sys/x86/iommu/intel_idpgtbl.c.

Reviewed by:	kib