posix_fallocate.3: Mention glibc emulation caveats

posix_fallocate.3: Mention glibc emulation caveats

When the glibc implementation of posix_fallocate detects
that the underlying filesystem does not support fallocate()
it uses an emulation function to attempt to allocate the
space requested. The most common case is calling
posix_fallocatei() for a file that is on NFS where the
NFS server is not new enough to support the recent fallocate
extensions. This emulation has various serious caveats that
must be understood in order to use posix_fallocate robustly
on all filesystems. The change documents the caveats in the
glibc implementation.

Lastly, we expand the meaning of EINVAL to match POSIX
2013 (Issue 7). If the underlying filesystem doesn't support
posix_fallocate()i, the implementation can return EINVAL, but
glibc does not do this, it emulates the operation instead.

Signed-off-by: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>