eCryptfs: Flush dirty pages in setattr

After 57db4e8d changed eCryptfs to
write-back caching, eCryptfs page writeback updates the lower inode
times due to the use of vfs_write() on the lower file.

To preserve inode metadata changes, such as 'cp -p' does with
utimensat(), we need to flush all dirty pages early in
ecryptfs_setattr() so that the user-updated lower inode metadata isn't
clobbered later in writeback.

https://bugzilla.kernel.org/show_bug.cgi?id=33372



Reported-by: Rocko <rockorequin@hotmail.com>
Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>