nfsd: allow for up to 32 callback session slots

nfsd currently only uses a single slot in the callback channel, which is
proving to be a bottleneck in some cases. Widen the callback channel to
a max of 32 slots (subject to the client's target_maxreqs value).

Change the cb_holds_slot boolean to an integer that tracks the current
slot number (with -1 meaning "unassigned").  Move the callback slot
tracking info into the session. Add a new u32 that acts as a bitmap to
track which slots are in use, and a u32 to track the latest callback
target_slotid that the client reports. To protect the new fields, add
a new per-session spinlock (the se_lock). Fix nfsd41_cb_get_slot to always
search for the lowest slotid (using ffs()).

Finally, convert the session->se_cb_seq_nr field into an array of
ints and add the necessary handling to ensure that the seqids get
reset when the slot table grows after shrinking.

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