fs: don't take the i_lock in inode_inc_iversion

The rationale for taking the i_lock when incrementing this value is
lost in antiquity. The readers of the field don't take it (at least
not universally), so my assumption is that it was only done here to
serialize incrementors.

If that is indeed the case, then we can drop the i_lock from this
codepath and treat it as a atomic64_t for the purposes of
incrementing it. This allows us to use inode_inc_iversion without
any danger of lock inversion.

Note that the read side is not fetched atomically with this change.
The assumption here is that that is not a critical issue since the
i_version is not fully synchronized with anything else anyway.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>