LU-13745 llite: switch generic_file_splice_read() to use of ->read_iter()

LU-13745 llite: switch generic_file_splice_read() to use of ->read_iter()

... and kill the ->splice_read() instances that can be switched to it

* Special Note * Using generic_file_splice_read() breaks Lustre
but in my testing on RHEL7 not provding a ->splice_read, which
results in default_file_splice_read() to be used seems to work.

Linux-commit: 82c156f853840645604acd7c2cebcb75ed1b6652

Change-Id: I28bd2fe43fec25e881c78b41c58c59ac5e74eb49
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-on: https://review.whamcloud.com/39272
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>