overlay: Call ovl_drop_write() earlier in ovl_dentry_open()

[ Upstream commit f25801ee4680ef1db21e15c112e6e5fe3ffe8da5 ]

Call ovl_drop_write() earlier in ovl_dentry_open() before we call vfs_open()
as we've done the copy up for which we needed the freeze-write lock by that
point.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>