Only sleep interruptible while waiting for suspension end when

Only sleep interruptible while waiting for suspension end when
filesystem specified VFCF_SBDRY flag, i.e. for NFS.

There are two issues with the sleeps.  First, applications may get
unexpected EINTR from the disk i/o syscalls.  Second, interruptible
sleep allows the stop of the process, and since mount point is
referenced while thread sleeps, unmount cannot free mount point
structure' memory, blocking unmount indefinitely.

Even for NFS, it is probably only reasonable to enable PCATCH for intr
mounts, but this information is currently not available at VFS level.

Reported and tested by:	pho (previous version)
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week