vfs: Fold follow_mount_rcu() into follow_dotdot_rcu()

This is needed before commit 4023bfc9f351 ('be careful with nd->inode
in path_init() and follow_dotdot_rcu()').  A similar change was made
upstream as part of commit b37199e626b3 ('rcuwalk: recheck mount_lock
after mountpoint crossing attempts').

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --cc fs/namei.c
Simple merge