Hi,

I have found a performance issue with the sort command when used on
pseudo files with zero size. For instance, sorting `/proc/kallsyms`, as
demonstrated below, takes significantly longer than executing with
`cat`, generating numerous temporary files. I confirmed this issue on
v8.32 as well as on commit 8f3989d in the master branch.

   $ time cat /proc/kallsyms | sort > /dev/null
   real    0m0.954s
   user    0m0.873s
   sys     0m0.096s

   $ time sort /proc/kallsyms > /dev/null
   real    0m8.555s
   user    0m3.367s
   sys     0m5.064s

   $ strace -e trace=openat sort /proc/kallsyms 2>&1 > /dev/null \
     | grep /tmp/sort | head -100
   ...
   openat(AT_FDCWD, "/tmp/sortM6Y6Y1", ...
   openat(AT_FDCWD, "/tmp/sortPrHKMG", ...

   $ strace -e trace=openat -c sort /proc/kallsyms > /dev/null
   % time     seconds  usecs/call     calls    errors syscall
   ------ ----------- ----------- --------- --------- ----------------
   100.00    6.419777          19    333258         8 openat
   ------ ----------- ----------- --------- --------- ----------------
   100.00    6.419777          19    333258         8 total

It appears that the buffer size allocated for pseudo files with zero
size is insufficient, likely because it is based on their file size,
which is zero. As seen in the attached patch, I think using
`INPUT_FILE_SIZE_GUESS` to calculate the buffer size when the file size
is zero would resolve this issue.

Best regards,
Takashi Kusumi