to #23113286

Since the stacking of regular file operations [1], the overlayfs edition of
write_iter() is called when writing regular files.

Since then, xattr lookup is needed on every write since file_remove_privs()
is called from ovl_write_iter(), which would become the performance
bottleneck when writing small chunks of data. In my test case,
file_remove_privs() would consume ~15% CPU when running fstime of unixbench
(the workload is repeadly writing 1 KB to the same file) [2].

Inherit the SB_NOSEC flag from upperdir. Since then xattr lookup would be
done only once on the first write. Unixbench fstime gets a ~20% performance
gain with this patch.

[1] https://lore.kernel.org/lkml/20180606150905.GC9426@magnolia/T/
[2] https://www.spinics.net/lists/linux-unionfs/msg07153.htmlSigned-off-by: NJeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>
Link: https://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs.git/commit/?h=overlayfs-next&id=b6dee44c57c785a59ef5f1f71588d13ebd89d395Acked-by: NJoseph Qi <joseph.qi@linux.alibaba.com>