nfs: fix PNFS_FLEXFILE_LAYOUT Kconfig default

stable inclusion
from stable-5.10.27
commit 2479c6b9ef36f9fd7ed37dbeb8f220bdeb175710
bugzilla: 51493

--------------------------------

[ Upstream commit a0590473 ]

This follows what was done in 8c2fabc6.
With the default being m, it's impossible to build the module into the
kernel.

Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Acked-by:   Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>