NFSv4.2: Fix a race in nfs42_proc_deallocate()

When punching holes in a file, we want to ensure the operation is
serialised w.r.t. other writes, meaning that we want to call
nfs_sync_inode() while holding the inode lock.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>