d_set_mounted(): we don't need to bump seqcount component of rename_lock

d_set_mounted(): we don't need to bump seqcount component of rename_lock

IOW, read_seqlock_excl() is sufficient there; no need to bother
with write_seqlock() (forcing all rename_lock readers into retry).

That leaves rename_lock taken for write only when we want to change
someone's parent or name.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>