iov_iter: Add missing speculation barrier to copy_from_user_iter()

iov_iter: Add missing speculation barrier to copy_from_user_iter()

The results of "access_ok()" can be mis-speculated.  The result is that
the CPU can end speculatively:

	if (access_ok(from, size))
		// Right here

For the same reason as done in copy_from_user() in commit 74e19ef0ff80
("uaccess: Add speculation barrier to copy_from_user()"), add a speculation
barrier to copy_from_user_iter().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/6b73e69cc7168c89df4eab0a216e3ed4cca36b0a.1763396724.git.christophe.leroy@csgroup.eu