f2fs: skip inode folio lookup for cached overwrite

f2fs: skip inode folio lookup for cached overwrite

prepare_write_begin() first gets the inode folio and builds a dnode,
then checks the read extent cache. For an ordinary overwrite of a
non-inline and non-compressed file, an extent-cache hit already gives the
data block address and the following path does not need to allocate or
update any node state.

Check the read extent cache before fetching the inode folio for that
narrow case. Keep the existing paths for inline data, compressed files,
and writes that may extend past EOF, where the helper may need inline
conversion, compression preparation, or block reservation.

This avoids a node-folio lookup in the buffered overwrite fast path when
the mapping is already cached.

In a QEMU/KASAN x86_64 VM, using a small buffered overwrite workload on
an existing 1MiB file, median time improved as follows:

  64-byte overwrites:  1724.93 ns/write -> 1560.24 ns/write
  256-byte overwrites: 1713.38 ns/write -> 1577.85 ns/write

Function profiling of 20k 64-byte overwrites showed
f2fs_get_inode_folio() calls drop from 20004 to 4.

Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>