Unionfs: bugfix -- set copied up open file mode to read and write

Unionfs: bugfix -- set copied up open file mode to read and write

When we copyup a file, we used to set the new file's open mode to write
only.  But we need read permission too.  We need write permission because
we're creating a new file; and we need read permission because the original
file had to have been readable.  If we don't set the new files permission to
BOTH read and write, we get -EBADF when trying calling vfs_read/vfs_write on
a copied up file.  (This fix is somewhat related to unionfs_readpage fixes
that use vfs_read instead of calling the lower readpage.)

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>