target/file: Do not return error for UNMAP if length is zero
The function fd_execute_unmap() in target_core_file.c calles
ret = file->f_op->fallocate(file, mode, pos, len);
Some filesystems implement fallocate() to return error if
length is zero (e.g. btrfs) but according to SCSI Block
Commands spec UNMAP should return success for zero length.
Signed-off-by: Jiang Yi <jiangyilism@gmail.com> Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>