btrfs: lift some btrfs_cross_ref_exist checks in nocow path

In nocow path, we check if the extent is snapshotted in
btrfs_cross_ref_exist(). We can do the similar check earlier and avoid
unnecessary search into extent tree.

A fio test on a Intel D-1531, 16GB RAM, SSD RAID-5 machine as follows:

[global]
group_reporting
time_based
thread=1
ioengine=libaio
bs=4k
iodepth=32
size=64G
runtime=180
numjobs=8
rw=randwrite

[file1]
filename=/mnt/nocow/testfile

IOPS result:   unpatched     patched

1 fio round:     46670        46958
snapshot
2 fio round:     51826        54498
3 fio round:     59767        61289

After snapshot, the first fio get about 5% performance gain. As we
continually write to the same file, all writes will resume to nocow mode
and eventually we have no performance gain.

Signed-off-by: Ethan Lien <ethanlien@synology.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update comments ]
Signed-off-by: David Sterba <dsterba@suse.com>