xfs_io: fix mread with length 1 mod page size

xfs_io: fix mread with length 1 mod page size

Fix a weird bug in mread where if you passed it a length that was 1
modulo the page size, for example

        xfs_io -r file -c "mmap -r 0 8192" -c "mread -v 0 4097"

... it never reset its pointer into the buffer into which it copies the
data from the memory map.  This caused an out-of-bounds write, which
depending on the length passed could be very large and reliably
segfault.  Also nothing was printed, despite the use of -v option.

(I don't know if this case gets reached by any existing xfstest, but
presumably not.  I noticed it while working on a patch to an xfstest.)

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>