[PATCH] procfs: expose umask in /proc/<PID>/status (formerly umask2, formerly getumask)

[Richard W.M. Jones]

It's not possible to read the process umask without also modifying it,
which is what umask(2) does.  A library cannot read umask safely,
especially if the main program might be multithreaded.

Add a new status line ("Umask") in /proc/<PID>/status.  It contains
the file mode creation mask (umask) in octal.  It is only shown for
tasks which have task->fs.

For the library this allows me to read the umask from
/proc/self/status.

This patch is adapted from one originally written by Pierre Carrier:
https://lkml.org/lkml/2012/5/4/451

Rich.