fs/ntfs3: fold file size handling into ntfs_set_size()

fs/ntfs3: fold file size handling into ntfs_set_size()

Remove the separate ntfs_extend() and ntfs_truncate() helpers and route
file size changes through ntfs_set_size().

This consolidates ntfs3 size updates in one place and lets the write,
fallocate, and setattr paths share the same logic for updating i_size,
valid data length, and preallocated extents.

This patch fixes a few issues found during internal tests.

Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>