scsi: target: tcmu: Optimize queue_cmd_ring()

queue_cmd_ring() needs to check whether there is enough space in cmd ring
and data area for the cmd to queue.

Currently the sequence is:

 1) Calculate size the cmd will occupy on the ring based on estimation of
    needed iovs.

 2) Check whether there is enough space on the ring based on size from 1)

 3) Allocate buffers in data area.

 4) Calculate number of iovs the command really needs while copying
    incoming data (if any) to data area.

 5) Re-calculate real size of cmd on ring based on real number of iovs.

 6) Set up possible padding and cmd on the ring.

Step 1) must not underestimate the cmd size so use max possible number of
iovs for the given I/O data size. The resulting overestimation can be
really high so this sequence is not ideal. The earliest the real number of
iovs can be calculated is after data buffer allocation. Therefore rework
the code to implement the following sequence:

 A) Allocate buffers on data area and calculate number of necessary iovs
    during this.

 B) Calculate real size of cmd on ring based on number of iovs.

 C) Check whether there is enough space on the ring.

 D) Set up possible padding and cmd on the ring.

The new sequence enforces the split of new function tcmu_alloc_data_space()
from is_ring_space_avail(). Using this function, change queue_cmd_ring()
according to the new sequence.

Change routines called by tcmu_alloc_data_space() to allow calculating and
returning the iov count. Remove counting of iovs in scatter_data_area().

Link: https://lore.kernel.org/r/20200910155041.17654-3-bstroesser@ts.fujitsu.com
Acked-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>