wait for iput_async before entering evict_inodes in generic_shutdown_super.
The reason we must finish before evict_inodes is iput when lazytime is on or
when zfs_purgedir calls zfs_zget would bump i_count from 0 to 1. This would
race with the i_count check in evict_inodes, so it could destroy the inode
while we are still using it.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>

When zfs_domount fails zsb will be freed, and its caller
mount_nodev/get_sb_nodev will do deactivate_locked_super and calls into
zfs_preumount.

In order to make sure we don't touch any nonexistent stuff, we must make sure
s_fs_info is NULL in the fail path so zfs_preumount can easily check that.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>