NFSD: Correct type annotations in user xattr helpers

Squelch some sparse warnings:

/home/cel/src/linux/linux/fs/nfsd/vfs.c:2264:13: warning: incorrect type in assignment (different base types)
/home/cel/src/linux/linux/fs/nfsd/vfs.c:2264:13:    expected int err
/home/cel/src/linux/linux/fs/nfsd/vfs.c:2264:13:    got restricted __be32
/home/cel/src/linux/linux/fs/nfsd/vfs.c:2266:24: warning: incorrect type in return expression (different base types)
/home/cel/src/linux/linux/fs/nfsd/vfs.c:2266:24:    expected restricted __be32
/home/cel/src/linux/linux/fs/nfsd/vfs.c:2266:24:    got int err
/home/cel/src/linux/linux/fs/nfsd/vfs.c:2288:13: warning: incorrect type in assignment (different base types)
/home/cel/src/linux/linux/fs/nfsd/vfs.c:2288:13:    expected int err
/home/cel/src/linux/linux/fs/nfsd/vfs.c:2288:13:    got restricted __be32
/home/cel/src/linux/linux/fs/nfsd/vfs.c:2290:24: warning: incorrect type in return expression (different base types)
/home/cel/src/linux/linux/fs/nfsd/vfs.c:2290:24:    expected restricted __be32
/home/cel/src/linux/linux/fs/nfsd/vfs.c:2290:24:    got int err

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>