pramfs: persistent and protected RAM filesystem

Hi all,

third round for the patch series (I hope this is the last one :)). I summarize here the changes to improve the review:

v3:
- fix a possible memory leak in an error path reported by yidong zhang
- fix a warning when using XIP about not used __pram_mmap symbol
- fix test module header description and replaced TEST_MODULE with PRAMFS_TEST_MODULE in the Kconfig and Makefile according to the comments done by Randy Dunlap
- fix a compilation warning in super.c reported by James Hogan
- fix a compilation error when XIP was enabled
- removed not used symbol PRAM_XATTR_INDEX_LUSTRE
- fix some comment style issue

v2:
- fix documentation errors reported by Randy Dunlap and Kieran Bingham
- reworked memory write protection functions with the suggestions of Andi Kleen

v1:
- first draft

Marco