Disable mount(8) canonical paths in do_mount()

Disable mount(8) canonical paths in do_mount()

By default the mount(8) command, as invoked by 'zfs mount', will try
to resolve any path parameter in its canonical form: this could lead
to mount failures when the cwd contains a symlink having the same name
of the dataset being mounted.

Fix this by explicitly disabling mount(8) path canonicalization.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #1791
Closes #6429
Closes #6437