Avoid creating another copy when mmap-ing files on RAMFS filesystem

[wkozaczuk]

Existing implementation of vfs_file::mmap() creates extra copy of file data in memory when mmap() is called on ROFS or RAMFS file. The function default_file_mmap() uses map_file_page_read page provider which calls read() on a file and ends up copying the data when page fault happens.

Ideally we would rather point to the same memory area that RAMFS or ROFS loaded the data to. Very likely it would have to be solved differently for ROFS vs RAMFS. In case of RAMFS file already resides in memory when mmap is called. With ROFS file data is loaded into cache lazily so possibly the cache memory area could be allocated using mmap() instead of malloc() and then simply pointed to the same pages when mmap is called on a file.

[nyh]

Note that RAMFS files are not necessarily aligned to 4096 byte blocks (or to any other alignment). Starting with bf73b9a they may point directly to the bootfs dump stored in memory. If we want to allow those to be mmap()ed without copy, we will need to modify the bootfs format (see scripts/mkboofs.py and fs/vfs/main.cc) to pad large files to page-sized alignment. To reduce the size impact of this padding to a minimum, we should do this only for large files, and leave small files unpadded (and requiring a copy on mmap()).