vfat: simplify checks in vfat_lookup()

vfat_d_anon_disconn() is called only if alias->d_parent is equal to
dentry->d_parent *and* it returns false unless alias->d_parent == alias.
But in that case alias is the directory we are doing lookup in, and
d_splice_alias() would've done the right thing.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>