alpha: switch osf_mount() to strndup_user()

... same as native mount(2) is doing for devname argument.  While we
are at it, fix misspelling ufs_args as cdfs_args in osf_ufs_mount() -
layouts are identical, so it doesn't change anything, but the current
variant is confusing for no reason.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>