[msan] Fix getmntent{_r} for empty /etc/fstab

Some configuration (for instance default docker ubuntu images) uses
a default empty and invalid /etc/fstab configuration file.  It makes
any call to getmntent return NULL and it leads to failures on
Msan-aarch64{-with-call}-Test/MemorySanitizer.getmntent{_r}.

This patch fixes it by creating a temporary file with some valid
entries (although not valid for the system) to use along with
setmntent/getmntent.

llvm-svn: 302639