MFC 220791,220793,220846,221836,226364:

MFC 220791,220793,220846,221836,226364:
Add the posix_fallocate(2) syscall.  The default implementation in
vop_stdallocate() is filesystem agnostic and will run as slow as a
read/write loop in userspace; however, it serves to correctly
implement the functionality for filesystems that do not implement a
VOP_ALLOCATE.

Allow VOP_ALLOCATE to be iterative, and have kern_posix_fallocate(9)
drive looping and potentially yielding.

Reviewed by:	mdf

git-svn-id: svn://svn.freebsd.org/base/stable/8@227573 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f