LU-16258 llite: Explicitly support .splice_write

LU-16258 llite: Explicitly support .splice_write

Linux commit v5.9-rc1-6-g36e2c7421f02
  fs: don't allow splice read/write without explicit ops

Lustre supports splice_write and previously provide handlers
for splice_read.
Explicitly use iter_file_splice_write, if it exists.

Lustre-change: https://review.whamcloud.com/48928
Lustre-commit: c619b6d6a54235cc0e34a65cf5916a632f4011c3

HPE-bug-id: LUS-11259
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I858688fc9b4dd370b6018c3b134f01e580477b25
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/49047
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>