vfs_vnops.c: Make O_NAMEDATTR Solaris compatible

When the O_XATTR flag is used in a Solaris open(2),
the named attribute directory is created, if it does
not already exist. (Solaris does not require or allow
O_CREAT for this case.)

The consensus on the mailing list was that O_NAMEDATTR
should behave the same way, as in "create the named
attribute directory if it does not exist even if O_CREAT
is not specified.

This patch makes this change. Note that CREATENAMED is
still useful for the NFSv4 server.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D50139
Fixes: 2ec2ba7e232d ("vfs: Add VFS/syscall support for Solaris style extended attributes")