Btrfs: optimize key searches in btrfs_search_slot

When the binary search returns 0 (exact match), the target key
will necessarily be at slot 0 of all nodes below the current one,
so in this case the binary search is not needed because it will
always return 0, and we waste time doing it, holding node locks
for longer than necessary, etc.

Below follow histograms with the times spent on the current approach of
doing a binary search when the previous binary search returned 0, and
times for the new approach, which directly picks the first item/child
node in the leaf/node.

Current approach:

Count: 6682
Range: 35.000 - 8370.000; Mean: 85.837; Median: 75.000; Stddev: 106.429
Percentiles:  90th: 124.000; 95th: 145.000; 99th: 206.000
  35.000 -   61.080:  1235 ################
  61.080 -  106.053:  4207 #####################################################
 106.053 -  183.606:  1122 ##############
 183.606 -  317.341:   111 #
 317.341 -  547.959:     6 |
 547.959 - 8370.000:     1 |

Approach proposed by this patch:

Count: 6682
Range:  6.000 - 135.000; Mean: 16.690; Median: 16.000; Stddev:  7.160
Percentiles:  90th: 23.000; 95th: 27.000; 99th: 40.000
   6.000 -    8.418:    58 #
   8.418 -   11.670:  1149 #########################
  11.670 -   16.046:  2418 #####################################################
  16.046 -   21.934:  2098 ##############################################
  21.934 -   29.854:   744 ################
  29.854 -   40.511:   154 ###
  40.511 -   54.848:    41 #
  54.848 -   74.136:     5 |
  74.136 -  100.087:     9 |
 100.087 -  135.000:     6 |

These samples were captured during a run of the btrfs tests 001, 002 and
004 in the xfstests, with a leaf/node size of 4Kb.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>