Looking up mount options can be very expensive on servers with many
mounted file systems.  When doing "zfs get" with any "-s" option that
does not include "temporary", the mount list will never be used.  This
commit optimizes for that case.

This is a breaking commit for libzfs!  Callers of zfs_get_prop are now
required to initialize src.  To preserve existing behavior, they should
initialize it to ZPROP_SRC_NONE.

Sponsored by: Axcient
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alan Somers <asomers@gmail.com>
Closes #11955