nfs: fix PNFS_FLEXFILE_LAYOUT Kconfig default

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>