Fix 'zfs remap <poolname@snapname>'

Fix 'zfs remap <poolname@snapname>'

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.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7454