security: protect legacy applications from executing with insufficient privilege

When cap_bset suppresses some of the forced (fP) capabilities of a file,
it is generally only safe to execute the program if it understands how to
recognize it doesn't have enough privilege to work correctly.  For legacy
applications (fE!=0), which have no non-destructive way to determine that
they are missing privilege, we fail to execute (EPERM) any executable that
requires fP capabilities, but would otherwise get pP' < fP.  This is a
fail-safe permission check.

For some discussion of why it is problematic for (legacy) privileged
applications to run with less than the set of capabilities requested for
them, see:

 http://userweb.kernel.org/~morgan/sendmail-capabilities-war-story.html

With this iteration of this support, we do not include setuid-0 based
privilege protection from the bounding set.  That is, the admin can still
(ab)use the bounding set to suppress the privileges of a setuid-0 program.

[[email protected]: coding-style fixes]
[[email protected]: cleanup]
Signed-off-by: Andrew G. Morgan <[email protected]>
Acked-by: Serge Hallyn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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