[XFS] fix error inversion problems with data flushing

XFS gets the sign of the error wrong in several places when
gathering the error from generic linux functions. These functions
return negative error values, while the core XFS code returns
positive error values. Hence when XFS inverts the error to be
returned to the VFS, it can incorrectly invert a negative
error and this error will be ignored by the syscall return.

Fix all the problems related to calling filemap_* functions.

Problem initially identified by Nick Piggin in xfs_fsync().

Signed-off-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Niv Sardi <xaiki@sgi.com>