nfs: do not export discarded symbols

The function nfs4_pnfs_v3_ds_connect_unload is exported so it can be
used by other modules, but is also marked '__exit' and will be
discarded when built into the kernel, as pointed out by this
linker error:

`nfs4_pnfs_v3_ds_connect_unload' referenced in section `___ksymtab_gpl+nfs4_pnfs_v3_ds_connect_unload' of fs/built-in.o: defined in discarded section `.exit.text' of fs/built-in.o

This removes the __exit annotation to make it safe to call this function.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 5f01d953 ("nfs41: create NFSv3 DS connection if specified")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>