Don't optimise bfd_seek to same position

It's not worth avoiding an fseek to the same position, and can cause
problems if the linker's output file (which is opened "w+") is read,
because that can result in writing, reading, then writing again.

POSIX.1-2017 (IEEE Std 1003.1) says of fopen:
"When a file is opened with update mode ('+' as the second or third
character in the mode argument), both input and output may be
performed on the associated stream. However, the application shall
ensure that output is not directly followed by input without an
intervening call to fflush() or to a file positioning function
(fseek(), fsetpos(), or rewind()), and input is not directly followed
by output without an intervening call to a file positioning function,
unless the input operation encounters end-of-file."

	* bfdio.c (bfd_seek): Always call iovec->bseek.