[vboxsf] speedup: remove redundant statting

There's no need to force a restat right after filling a new inode with stat
information. Also, open() can use the stat info it gets with the handle
instead of statting again.

In addition, sf_inode_revalidate should actually set force_restat back to 0
when statting.
[vboxsf] bugfix: make sure writepage still works after concurrent open/close
[vboxsf] speed up reads by using multipage transfers
[vboxsf] support writable mmap

This is mainly so that guest processes can use the shared folder as a backing
for database files and such. Don't expect coherence if a file is simultaneously
mmapped on the host and guest sides.
[vboxsf] bugfix: set handle read/write even if some bits already set

In sf_reg_open(), a file opened with O_TRUNC|O_RDWR wouldn't get the
SHFL_CF_ACCESS_READ flag because of the logic that skipped processing
the O_RDWR after seeing O_TRUNC. This is wrong, and it shows up with
the new pagecache code because the handle is not flagged as readable.
[vboxsf] speedup: satisfy read() from page cache if possible
[vboxsf] speedup: implement readpages() for bulk reads
[packaging] Move patches to git commits; remove unneeded patches