ovl: fetch inode once in ovl_dentry_revalidate_common()

d_inode_rcu() is right - we might be in rcu pathwalk;
however, OVL_E() hides plain d_inode() on the same dentry...

Fixes: a6ff2bc0 ("ovl: use OVL_E() and OVL_E_FLAGS() accessors")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>