Fixed sendfile() limit handling on Linux.

Fixed sendfile() limit handling on Linux.

On Linux starting with 2.6.16, sendfile() silently limits all operations
to MAX_RW_COUNT, defined as (INT_MAX & PAGE_MASK).  This incorrectly
triggered the interrupt check, and resulted in 0-sized writev() on the
next loop iteration.

Fix is to make sure the limit is always checked, so we will return from
the loop if the limit is already reached even if number of bytes sent is
not exactly equal to the number of bytes we've tried to send.