MFV r337161: 9512 zfs remap poolname@snapname coredumps

MFV r337161: 9512 zfs remap poolname@snapname coredumps

Only filesystems and volumes are valid "zfs remap" parameters: when passed
a snapshot name zfs_remap_indirects() does not handle the EINVAL returned
from libzfs_core, which results in failing an assertion and consequently
crashing.

illumos/illumos-gate@0b2e8253986c5c761129b58cfdac46d204903de1

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: John Wren Kennedy <john.kennedy@delphix.com>
Reviewed by: Sara Hartse <sara.hartse@delphix.com>
Approved by: Matt Ahrens <mahrens@delphix.com>
Author:     loli10K <ezomori.nozomu@gmail.com>