Patch from: Oleg Drokin <green@namesys.com>

It moves all the arg checking code from the start of generic_file_aio_write()
into a standalone function, so other filesystems can avoid having to
cut-n-paste them.

The new function is exported to modules, and also inlined in filemap.c so
that the current filesystems are unaffected.  If someone is using ext2 and
reiserfs at the same time, they lose a bit of icache.