Currently sys_read/sys_pread and sys_write/sys_pwrite basically contain
lots of duplication of the same checks/code.

This moves all that into vfs_read/vfs_write helpers that have the same
prototypes as the read/write file operations.  In addition I have
choosen to export these interfaces to module so people doing inkernel
file reading/writing can use these instead of duplicating the checks
(which is very likely to be done wrong).