generic/759,760: fix MADV_COLLAPSE detection and inclusion

generic/759,760: fix MADV_COLLAPSE detection and inclusion

On systems with "old" C libraries such as glibc 2.36 in Debian 12, the
MADV_COLLAPSE flag might not be defined in any of the header files
pulled in by sys/mman.h, which means that the fsx binary might not get
built with any of the MADV_COLLAPSE code.  If the kernel supports THP,
the test will fail with:

>  QA output created by 760
>  fsx -N 10000 -l 500000 -r PSIZE -t BSIZE -w BSIZE -Z -R -W -h
> -fsx -N 10000 -o 8192 -l 500000 -r PSIZE -t BSIZE -w BSIZE -Z -R -W -h
> -fsx -N 10000 -o 128000 -l 500000 -r PSIZE -t BSIZE -w BSIZE -Z -R -W -h
> +mapped writes DISABLED
> +MADV_COLLAPSE not supported. Can't support -h

Fix both tests to detect fsx binaries that don't support MADV_COLLAPSE,
then fix fsx.c to include the mman.h from the kernel headers (aka
linux/mman.h) so that we can actually test IOs to and from THPs if the
kernel is newer than the rest of userspace.

Cc: <fstests@vger.kernel.org> # v2025.02.02
Cc: joannelkoong@gmail.com
Fixes: 627289232371e3 ("generic: add tests for read/writes from hugepages-backed buffers")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>