uucore/fsext: read_fs_list might get stuck in an infinite loop

[tertsdiepraam]

Similar to: #4539

On Linux, read_fs_list opens either /proc/self/mountinfo or /etc/mtab depending on which it can find and reads the list from the lines in that file. However, filter_map(|line| line.ok()) is used, which will silently ignore errors and might cause an infinite loop. Now this only happens when these specific files can be opened but not read (i.e. when they are a directory), so this is extremely unlikely. Still I think this should be fixed. If anyone can come up with a reliable testcase where this happens it would be much appreciated :)