Fix link count of root inode when snapdir is visible

Given how zfs_getattr() is implemented, zfs_getattr_fast() (used by
->getattr() of zpl inodes) also needs to consider an additional link
count if "snapdir" property is set to "visible".

Without this, # of directories in root inode of each dataset doesn't
match the link count when snapdir is visible.

Reviewed-by: Richard Yao <ryao@gentoo.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #8727