nfscommon: Factor out conversion of ae_perm to NFSv4 ACE flags

This patch factors the code that converts the ae_perm bits in
an NFSv4 style ACL to the NFSv4 on-the-wire format into a
separate function.

This function will be used by the NFSv4 server in a future
commit.

This patch should not change NFSv4 semantics.