Only reduce __zio_execute() stack usage in kernel space

Related to 91579709fccd3e55a21970742b66c388fb1403db we need to
be very careful about not overrunning the stack in kernel space.
However, in user space we're already allowing slightly larger
stacks so this stack usage optimization is not required there.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Revert "Avoid ELOOP on auto-mounted snapshots"

This reverts commit 7afcf5b1da83549bfba70a61fae7a00eaa63c2b0 which
accidentally introduced a regression with the .zfs snapshot directory.
While the updated code still does correctly mount the requested
snapshot.  It updates the vfsmount such that it references the
original dataset vfsmount.  The result is that the snapshot itself
isn't visible.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #816