binfmt_elf: Only report padzero() errors when PROT_WRITE

Errors with padzero() should be caught unless we're expecting a
pathological (non-writable) segment. Report -EFAULT only when PROT_WRITE
is present.

Additionally add some more documentation to padzero(), elf_map(), and
elf_load().

Cc: Eric Biederman <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: [email protected]
Cc: [email protected]
Suggested-by: Eric Biederman <[email protected]>
Tested-by: Pedro Falcato <[email protected]>
Signed-off-by: Sebastian Ott <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

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