pramfs: persistent and protected RAM filesystem

Hi all,

fifth round for the patch series. I summarize here the changes to improve the review:

v5:
- removed the changelog from documentation file
- added the function pram_check_flags in the file operations
- added a check when the user uses XIP and the blocksize is different from page size
- added i_meta_mutex to avoid race conditions in the inode update path
- changed the lock policy during mem{lock|unlock} operations
- replaced bitmap_set with bitmap_fill
- added mount options xip, acl, noacl, noprotect, user_xattr and nouser_xattr as required by Paul Maundt
- inserted pr_fmt in pram.h
- inserted macros IF2DT and DT2IF
- removed file name description from each file
- used min_t instead of using own check in pram_add_link()

v4:
- in bitmap init used already present bitmap_set function
- fix a possible memory leak in an error path reported by yidong zhang

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