LU-14043 lfs: use buffered IO in mirror extend

LU-14043 lfs: use buffered IO in mirror extend

In lfs_mirror_extend() use buffered IO to read from the source
file. Mirror resync will continue to use direct IO on the source.
Call posix_fadvise() at the end of mirroring to discard cached data
from the old and new mirrors.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I7703ae9e30cf30a7e6fbf274114d5986a80d4d3a
Reviewed-on: https://review.whamcloud.com/40275
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Patrick Farrell <farr0186@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>