exfat: simplify exfat_lookup()

1) d_splice_alias() handles ERR_PTR() for inode just fine
2) no need to even look for existing aliases in case of directory inodes;
   just punt to d_splice_alias(), it'll do the right thing
3) no need to bother with 'd_unhashed(alias)' case - d_find_alias()
   would've returned that only in case of a directory, and d_splice_alias()
   will handle that just fine on its own.
4) exfat_d_anon_disconn() is entirely pointless now - we only get to
   evaluating it in case dentry->d_parent == alias->d_parent and
   alias being a non-directory.  But in that case IS_ROOT(alias) can't
   possibly be true - that would've reqiured alias == alias->d_parent,
   i.e alias == dentry->d_parent and dentry->d_parent is guaranteed to
   be a directory.  So exfat_d_anon_disconn() would always return false
   when it's called, which makes && !exfat_d_anon_disconn(alias)
   a no-op.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>