daxfs: Fix truncate not updating VFS i_size in setattr

[souvik1997]

daxfs_setattr() relied on setattr_copy() to update i_size when handling ATTR_SIZE (e.g. O_TRUNC). However, setattr_copy() does not handle ATTR_SIZE — the filesystem must update i_size itself.

This caused a bug where overwriting an existing file via shell redirect (echo "..." > file) would produce empty reads:

1. O_TRUNC triggers setattr(ATTR_SIZE=0) which creates an overlay inode entry with size=0, but VFS i_size stays at the original base image size (e.g. 23 bytes)
2. The subsequent write (21 bytes) sees pos(21) <= i_size(23), so the "update size if extending" check is skipped — the overlay inode size is never updated from 0
3. On read, daxfs_refresh_isize() reads overlay size=0 and clobbers VFS i_size to 0, causing the read to return empty

Fix by calling truncate_setsize() before setattr_copy() when ATTR_SIZE is set. This is the standard pattern used by other Linux filesystems (ramfs, tmpfs, etc.).

Fixes: #8

[congwang-mk]

For long term, we may need to restructure daxfs_setattr().