vfs: add a O_NOMTIME flag

vfs: add a O_NOMTIME flag

Add the O_NOMTIME flag which prevents mtime from being updated which can
greatly reduce the IO overhead of writes to allocated and initialized
regions of files.

ceph servers can have loads where they perform O_DIRECT overwrites of
allocated file data and then sync to make sure that the O_DIRECT writes
are flushed from write caches.  If the writes dirty the inode with mtime
updates then the syncs also write out the metadata needed to track the
inodes which can add significant iop and latency overhead.

The ceph servers don't use mtime at all.  They're using the local file
system as a backing store and any backups would be driven by their upper
level ceph metadata.  For ceph, slow IO from mtime updates in the file
system is as daft as if we had block devices slowing down IO for
per-block write timestamps that file systems never use.

In simple tests a O_DIRECT|O_NOMTIME overwriting write followed by a
sync went from 2 serial write round trips to 1 in XFS and from 4 serial
IO round trips to 1 in ext4.

file_update_time() checks for O_NOMTIME and aborts the update if it's
set, just like the current check for the in-kernel inode flag
S_NOCMTIME.  I didn't update any other mtime update sites. They could be
added as we decide that it's appropriate to do so.

I opted not to name the flag O_NOCMTIME because I didn't want the name
to imply that ctime updates would be prevented for other inode changes
like updating i_size in truncate.  Not updating ctime is a side-effect
of removing mtime updates when it's the only thing changing in the
inode.

The criteria for using O_NOMTIME is the same as for using O_NOATIME:
owning the file or having the CAP_FOWNER capability.  If we're not
comfortable allowing owners to prevent mtime/ctime updates then we
should add a tunable to allow O_NOMTIME.  Maybe a mount option?

Signed-off-by: Zach Brown <zab@redhat.com>
Cc: Sage Weil <sweil@redhat.com>