configfs_detach_prep(): pass configfs_dirent instead of dentry

configfs_detach_prep(): pass configfs_dirent instead of dentry

The only thing it uses the argument for is its ->d_fsdata and
all callers have that already available.

Note that in the recursive call we are dealing with a (sub)directory
configfs_dirent, and for those ->s_dentry->d_fsdata points back
to configfs_dirent itself.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>