The functions btrfs_tree_lock() and btrfs_tree_read_lock() are very
trivial so that can be made inline and avoid call overhead, as they
are very often called inside critical sections (when searching a btree
for example, attempting to lock a child node/leaf while holding a lock
on the parent).

So make them static inline, which even reduces the size of the btrfs
module a little bit.

Before this change:

   $ size fs/btrfs/btrfs.ko
      text	   data	    bss	    dec	    hex	filename
   1718786	 156276	  16920	1891982	 1cde8e	fs/btrfs/btrfs.ko

After this change:

   $ size fs/btrfs/btrfs.ko
      text	   data	    bss	    dec	    hex	filename
   1718650	 156260	  16920	1891830	 1cddf6	fs/btrfs/btrfs.ko

Running fs_mark also showed a tiny improvement with this script:

   $ cat test.sh
   #!/bin/bash

   DEV=/dev/nullb0
   MNT=/mnt/nullb0
   FILES=100000
   THREADS=$(nproc --all)

   echo "performance" | \
       tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

   umount $DEV &> /dev/null
   mkfs.btrfs -f $DEV
   mount $DEV $MNT

   OPTS="-S 0 -L 5 -n $FILES -s 0 -t $THREADS -k"
   for ((i = 1; i <= $THREADS; i++)); do
        OPTS="$OPTS -d $MNT/d$i"
   done

   fs_mark $OPTS

   umount $MNT

Before this change:

   FSUse%        Count         Size    Files/sec     App Overhead
       10      1200000            0     180894.0         10705410
       16      2400000            0     228211.4         10765738
       23      3600000            0     215969.6         11011072
       30      4800000            0     199077.1         11145587
       46      6000000            0     176624.1         11658470

After this change:

   FSUse%        Count         Size    Files/sec     App Overhead
       10      1200000            0     185312.3         10708377
       16      2400000            0     229320.4         10858013
       23      3600000            0     217958.7         11006167
       30      4800000            0     205122.9         11112899
       46      6000000            0     178039.1         11438852
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>