MFV r316876: 7542 zfs_unmount failed with EZFS_UNSHARENFSFAILED

illumos/illumos-gate@09c9e6dc9b69d10b771bb87e01040ec320a0bfd3
https://github.com/illumos/illumos-gate/commit/09c9e6dc9b69d10b771bb87e01040ec320a0bfd3

https://www.illumos.org/issues/7542

libshare keeps a cached copy of the sharetab listing in memory, which can
become out of date if shares are destroyed or created while leaving a libzfs
handle open. This results in a spurious unmounting failure when an NFS share
exists but isn't in the stale libshare cache.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Matt Amdur <matt.amdur@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Chris Williamson <chris.williamson@delphix.com>