nfs: use locks_inode_context helper

nfs currently doesn't access i_flctx safely. This requires a
smp_load_acquire, as the pointer is set via cmpxchg (a release
operation).

Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Anna Schumaker <anna@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jeff Layton <jlayton@kernel.org>