This patch replaces the ctrl->namespaces tracking from linked list to
xarray and improves the performance when accessing one namespce :-

XArray vs Default:-

IOPS and BW (more the better) increase BW (~1.8%):-
---------------------------------------------------

 XArray :-
  read:  IOPS=160k,  BW=626MiB/s  (656MB/s)(18.3GiB/30001msec)
  read:  IOPS=160k,  BW=626MiB/s  (656MB/s)(18.3GiB/30001msec)
  read:  IOPS=162k,  BW=631MiB/s  (662MB/s)(18.5GiB/30001msec)

 Default:-
  read:  IOPS=156k,  BW=609MiB/s  (639MB/s)(17.8GiB/30001msec)
  read:  IOPS=157k,  BW=613MiB/s  (643MB/s)(17.0GiB/30001msec)
  read:  IOPS=160k,  BW=626MiB/s  (656MB/s)(18.3GiB/30001msec)

Submission latency (less the better) decrease (~8.3%):-
-------------------------------------------------------

 XArray:-
  slat  (usec):  min=7,  max=8386,  avg=11.19,  stdev=5.96
  slat  (usec):  min=7,  max=441,   avg=11.09,  stdev=4.48
  slat  (usec):  min=7,  max=1088,  avg=11.21,  stdev=4.54

 Default :-
  slat  (usec):  min=8,   max=2826.5k,  avg=23.96,  stdev=3911.50
  slat  (usec):  min=8,   max=503,      avg=12.52,  stdev=5.07
  slat  (usec):  min=8,   max=2384,     avg=12.50,  stdev=5.28

CPU Usage (less the better) decrease (~5.2%):-
----------------------------------------------

 XArray:-
  cpu  :  usr=1.84%,  sys=18.61%,  ctx=949471,  majf=0,  minf=250
  cpu  :  usr=1.83%,  sys=18.41%,  ctx=950262,  majf=0,  minf=237
  cpu  :  usr=1.82%,  sys=18.82%,  ctx=957224,  majf=0,  minf=234

 Default:-
  cpu  :  usr=1.70%,  sys=19.21%,  ctx=858196,  majf=0,  minf=251
  cpu  :  usr=1.82%,  sys=19.98%,  ctx=929720,  majf=0,  minf=227
  cpu  :  usr=1.83%,  sys=20.33%,  ctx=947208,  majf=0,  minf=235.
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>