syscalls: add back legacy __NR_nfsservctl macro

The conversion from the old unistd.h file to syscall.tbl dropped the
nfsservctl macro. This one was handled inconsistently across architectures
in the original introduction of the syscall.tbl format, and I went the
other way on this.

The syscall was already gone in linux-3.1 before the current users
of the generic table (other than openrisc) first appeared, so nobody
could actally use it, but putting the number back helps for consistency
since there are build scripts that check the presence of all these
macros.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=2301919


Signed-off-by: Arnd Bergmann <arnd@arndb.de>