The purpose of this PR is to accepts fadvise ioctl from userland
to do read-ahead by demand.
It could dramatically improve sequential read performance especially
when primarycache is set to metadata or zfs_prefetch_disable is 1.
If the file is mmaped, generic_fadvise is also called for page cache
read-ahead besides dmu_prefetch.
Only POSIX_FADV_WILLNEED and POSIX_FADV_SEQUENTIAL are supported in
this PR currently.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Finix Yan <yancw@info2soft.com>
Closes#13694
