f2fs: support resizable tail section and unify pinned allocation

Currently, zoned block devices restrict pinned file allocations to
conventional zones at the beginning of the storage (before
first_seq_zone_segno), triggering range GC when conventional space is
exhausted.

On regular block devices, when preparing for future online filesystem
resizing (e.g. partition shrinking), pinned files must not be allocated
in the tail area that will be truncated, as pinned files cannot be
relocated by GC. Specifying the resizable tail area size (in sections)
allows uniform mount configuration across devices of different storage
capacities.

To support this, introduce a unified `pinned_area_max_secno` boundary
abstraction in `f2fs_sb_info`:
1. Add `-o resizable_tail_secno=%u` mount option to specify the number
   of sections at the tail of the filesystem reserved for resizing.
2. In `f2fs_fill_super()`, initialize `sbi->pinned_area_max_secno` as:
   min(MAIN_SECS(sbi) - resizable_tail_sec, zoned_max_sec).
3. In `get_new_segment()`, restrict segment allocation for pinned files
   (`pinning == true`) to `0 .. sbi->pinned_area_max_secno - 1`. If no
   free section is available in the pinned area, return -EAGAIN.
4. In `f2fs_allocate_pinning_section()`, unify the range GC trigger to
   run `f2fs_gc_range()` up to `sbi->pinned_area_max_secno` whenever
   `sbi->pinned_area_max_secno < MAIN_SECS(sbi)` and allocation
   returns -EAGAIN.
5. Expose `/sys/fs/f2fs/<dev>/pinned_area_max_secno` as a read-only
   sysfs node.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Sunmin Jeong <s_min.jeong@samsung.com>
Reviewed-by: Wenjie Qi <qiwenjie@xiaomi.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

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