LU-15122 osd-ldiskfs: Fix ASSERTION( iobuf->dr_rw == 0 ) with 64KB PAGE_SIZE

LU-15122 osd-ldiskfs: Fix ASSERTION( iobuf->dr_rw == 0 ) with 64KB PAGE_SIZE

During a writing, if there is a page can not be mapped to blocks
at once, it will cause "ASSERTION( iobuf->dr_rw == 0 )" crash
which leads by the overflow access of mapped blocks array.

This will happen on Arm platforms easily with 64KB PAGE_SIZE.
And will not happen on x86_64 platforms with 4KB PAGE_SIZE.
Because for 4KB block size, if PAGE_SIZE is 4KB, then i == 0
and blocks_left_page == 1. Which makes the inner loop each time
handle one block. Thus the outer loop condition "block_idx <
block_idx_end;" insures blocks[] array access not overflow.

Check the actual mapped count so that mapped blocks array
access will not overflow.

Fixes: 0271b17b80a8 ("LU-14134 osd-ldiskfs: reduce credits for new writing")
Change-Id: Icd46c04bea2d7930456840694d422758eebb4186
Signed-off-by: Xinliang Liu <xinliang.liu@linaro.org>
Reviewed-on: https://review.whamcloud.com/45288
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>