fallocate syscall interface deficiency

The fallocate syscall returns ENOSYS in case the filesystem does not support
the operation and expects the userlevel code to fill in.  This is good in
concept.

The problem is that the libc code for old kernels should be able to
distinguish the case where the syscall is not at all available vs not
functioning for a specific mount point.  As is this is not possible and we
always have to invoke the syscall even if the kernel doesn't support it.

I suggest the following patch.  Using EOPNOTSUPP is IMO the right thing to do.

Cc: Amit Arora <aarora@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>