NFSv4.1/flexfiles: Fix incorrect usage of pnfs_generic_mark_devid_invalid()

Unlike the files layout, flexfiles does not test for the NFS_DEVICEID_INVALID
flag. Instead it relies on NFS_DEVICEID_UNAVAILABLE.
Fix is to replace with nfs4_mark_deviceid_unavailable().

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>