libxfs: provide a memfd_create() wrapper if not present in libc

libxfs: provide a memfd_create() wrapper if not present in libc

Commit 1cb2e387 "libxfs: add xfile support" introduced
a use of the memfd_create() system call, included in version
xfsprogs v6.9.0.

This system call was introduced in kernel commit [1], first included
in kernel v3.17 (released on 2014-10-05).

The memfd_create() glibc wrapper function was added much later in
commit [2], first included in glibc version 2.27 (released on
2018-02-01).

This direct use memfd_create() introduced a requirement on
Kernel >= 3.17 and glibc >= 2.27.

There is old toolchains like [3] for example (which ships gcc 7.3.1,
glibc 2.25 and includes kernel v4.10 headers), that can still be used
to build newer kernels. Even if such toolchains can be seen as
outdated, they are still claimed as supported by recent kernel.
For example, Kernel v6.10.5 has a requirement on gcc version 5.1 and
greater. See [4].

When compiling xfsprogs v6.9.0 with a toolchain not providing the
memfd_create() syscall wrapper, the compilation fail with message:

    xfile.c: In function 'xfile_create_fd':
    xfile.c:56:7: warning: implicit declaration of function 'memfd_create'; did you mean 'timer_create'? [-Wimplicit-function-declaration]
      fd = memfd_create(description, MFD_CLOEXEC | MFD_NOEXEC_SEAL);
           ^~~~~~~~~~~~

    ../libxfs/.libs/libxfs.a(xfile.o): In function 'xfile_create_fd':
    /build/xfsprogs-6.9.0/libxfs/xfile.c:56: undefined reference to 'memfd_create'

In order to let xfsprogs compile in a wider range of configurations,
this commit adds a memfd_create() function check in autoconf configure
script, and adds a system call wrapper which will be used if the
function is not available. With this commit, the environment
requirement is relaxed to only kernel >= v3.17.

Note: this issue was found in xfsprogs integration in Buildroot [5]
using the command "utils/test-pkg -a -p xfsprogs", which tests many
toolchain/arch combinations.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9183df25fe7b194563db3fec6dc3202a5855839c
[2] https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=59d2cbb1fe4b8601d5cbd359c3806973eab6c62d
[3] https://releases.linaro.org/components/toolchain/binaries/7.3-2018.05/aarch64-linux-gnu/gcc-linaro-7.3.1-2018.05-x86_64_aarch64-linux-gnu.tar.xz
[4] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/Documentation/process/changes.rst?h=v6.10.5#n32
[5] https://buildroot.org/

Signed-off-by: Julien Olivain <ju.o@free.fr>
Reviewed-by: Christoph Hellwig <hch@lst.de>