xfs: fix a double completion for buffers on in-memory targets

xfs: fix a double completion for buffers on in-memory targets

__xfs_buf_submit calls xfs_buf_ioend when b_io_remaining hits zero.  For
in-memory buftargs b_io_remaining is never incremented from it's initial
value of 1, so this always happens.  Thus the extra call to xfs_buf_ioend
in _xfs_buf_ioapply causes a double completion.  Fortunately
__xfs_buf_submit is only used for synchronous reads on in-memory buftargs
due to the peculiarities of how they work, so this is mostly harmless and
just causes a little extra work to be done.

Fixes: 5076a6040ca1 ("xfs: support in-memory buffer cache targets")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>