ffs_read: Replace wrong check returned EFBIG with EOVERFLOW handling from POSIX:
ffs_read:
Replace wrong check returned EFBIG with EOVERFLOW handling from POSIX:

36708 [EOVERFLOW] The file is a regular file, nbyte is greater than 0, the
starting position is before the end-of-file, and the starting position is
greater than or equal to the offset maximum established in the open file
description associated with fildes.

ffs_write:
Replace u_int64_t cast with uoff_t cast which is more natural for types
used.

ffs_write & ffs_read:
Remove uio_offset and uio_resid checks for negative values, the caller
supposed to do it already. Add comments about it.

Reviewed by:    bde

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase