'zfs share -a' should handle 'canmount=noauto'

The 'zfs share -a' currently skips any filesystems which
have 'canmount=noauto' set. This behavior is unexpected since the
one would expect 'zfs share -a' to share any mounted filesystem
that has the 'sharenfs' property already set.

This changes the behavior of 'zfs share -a' to allow the sharing
of 'canmount=noauto' datasets if they are mounted.

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Don Brady <don.brady@delphix.com>
Reviewed-by: Prakash Surya <prakash.surya@delphix.com>
Signed-off-by: George Wilson <gwilson@delphix.com>
External-issue: DLPX-71313
Closes #10688