io_[gs]etxattr_prep(): just use getname()

getname_flags(pathname, LOOKUP_FOLLOW) is obviously bogus - following
trailing symlinks has no impact on how to copy the pathname from userland...

Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>