NFSD: Move "select FS_POSIX_ACL if NFSD_V4"

Clean up: since FS_POSIX_ACL is a non-visible boolean entry, it can be
selected safely under the NFSD_V4 entry (also a boolean).

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