f2fs: optimize f2fs_overwrite_io() for f2fs_iomap_begin

f2fs: optimize f2fs_overwrite_io() for f2fs_iomap_begin

When overwriting already allocated blocks, f2fs_iomap_begin() calls
f2fs_overwrite_io() to check block mappings. However,
f2fs_overwrite_io() iterates through all mapped blocks in the range,
which can be inefficient for fragmented files with large I/O requests.

This patch optimizes f2fs_overwrite_io() by adding a 'check_first'
parameter and introducing __f2fs_overwrite_io() helper. When called from
f2fs_iomap_begin(), we only check the first mapping to determine if the
range is already allocated, which is sufficient for setting
map.m_may_create.

This optimization significantly reduces the number of f2fs_map_blocks()
calls in f2fs_overwrite_io() when called from f2fs_iomap_begin(),
especially for fragmented files with large I/O requests.

Cc: stable@kernel.org
Fixes: 351bc761338d ("f2fs: optimize f2fs DIO overwrites")
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Reviewed-by: Sunmin Jeong <s_min.jeong@samsung.com>
Signed-off-by: Yeongjin Gil <youngjin.gil@samsung.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>