knfsd: don't allocate file_locks on the stack

struct file_lock is pretty large and really ought not live on the stack.
On my x86_64 machine, they're almost 200 bytes each.

    (gdb) p sizeof(struct file_lock)
    $1 = 192

...allocate them dynamically instead.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>