BACKPORT: erofs: tidy up z_erofs_do_read_page()

 - Fix a typo: spiltted => split;

 - Move !EROFS_MAP_MAPPED and EROFS_MAP_FRAGMENT upwards;

 - Increase `split` in advance to avoid unnecessary repeats.

Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230817082813.81180-4-hsiangkao@linux.alibaba.com



Bug: 318378021
Change-Id: I465fd33c7cbbe91d5da4b4ee2343a7b319534148
(cherry picked from commit e4c1cf52)
[dhavale: resolved small conflict in zdata.c in z_erofs_do_read_page()]
Signed-off-by: Sandeep Dhavale <dhavale@google.com>