[backport request][stable] NFSv4.x: fix incorrect return value in copy_file_range

[Yu Xu <xuyu@xxxxxxxxxxxxxxxxx>]

Hi,

I'm using kernel v4.19.y and find that v4.19.y fails in the latest 
xfstests generic/075, 091, 112, 127 and 263, which can be simply 
reproduced as follows:

# kernel 4.19.y
$ xfs_io -fc "copy_range -s 0 -d 1 -l 1 /mnt/nfs/file" /mnt/nfs/file
copy_range: Invalid argument

# kernel 5.1.0
$ xfs_io -fc "copy_range -s 0 -d 1 -l 1 /mnt/nfs/file" /mnt/nfs/file
# success

I notice that upstream (v5.1+) has already fixed this issue with:
1) commit 45ac486ecf2dc998e25cf32f0cabf2deaad875be
2) commit 0769663b4f580566ef6cdf366f3073dbe8022c39

But these patches do not cc stable@xxxxxxxxxxxxxxx (why? forgotten?). 
And will v4.19.y consider to backport these patches?


Thanks,
Yu