nfsd: replace CB_GETATTR_BUSY with NFSD4_CALLBACK_RUNNING

nfsd: replace CB_GETATTR_BUSY with NFSD4_CALLBACK_RUNNING

These flags serve essentially the same purpose and get set and cleared
at the same time. Drop CB_GETATTR_BUSY and just use
NFSD4_CALLBACK_RUNNING instead.

For this to work, we must use clear_and_wake_up_bit(), but doing that on
for other types of callbacks is wasteful. Declare a new NFSD4_CALLBACK_WAKE
flag in cb_flags to indicate that wake_up is needed, and only set that
for CB_GETATTRs.

Also, make the wait use a TASK_UNINTERRUPTIBLE sleep. This is done in
the context of an nfsd thread, and it should never need to deal with
signals.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>