Clip the settings for the NFS rsize, wsize mount options
to a power of 2. For non-power of 2 settings, intermittent
page faults have been reported. Although the bug that causes
these page faults/crashes has not been identified, it does
not appear to occur when rsize, wsize is a power of 2.

Reported by:	tcberner@gmail.com
MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273486 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f