nfs: use lock_two_nondirectories()

nfs: use lock_two_nondirectories()

Rather than open-coding this function call it to make intention clear
and to use "correct" nesting levels (parent and child are for
directories).

This is purely cosmetic with no expected change in behaviour.

Signed-off-by: NeilBrown <neil@brown.name>
Link: https://lore.kernel.org/r/174942091741.608730.3327223511347232829@noble.neil.brown.name
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>