LU-6142 llite: use d_is_symlink to test if dentry is a symlink

LU-6142 llite: use d_is_symlink to test if dentry is a symlink

Using d_is_symlink() is preferred to testing ->get_link or
->follow_link.

A recent patch made this work for foreign files/dirs by making sure
the entry type in d_flags is correct, so we can simplify the code in
ll_revalidate_dentry().

Fixes: 15d44e787e17 ("LU-12682 llite: fake symlink type of foreign file/dir")
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ie4c33ae1fb9a660ccbd50e2c70b6cde65cc9b990
Reviewed-on: https://review.whamcloud.com/41770
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>