FIOSEEKHOLE/FIOSEEKDATA: correct constistency for bmap-based implementation

[kib]

Summary

Writes on UFS through a mapped region do not allocate disk blocks in
holes immediately. The blocks are allocated when the pages are first
time paged out.

This breaks the algorithm in vn_bmap_seekhole() and ufs_bmap_seekdata(),
because VOP_BMAP() report hole for the place which already contains a
valid data.

Clean pages before doing VOP_BMAP() in the affected functions.  In
principle, we could clean less by only requesting clean starting from
the offset, but it is probably not very important.

PR:     269261
Reported by:    asomers

[kib]

Zap unneeded VOP_FDATASYNC

[markj]

sys/kern/vfs_vnops.c

2568

Why exactly do we need the exclusive lock? VOP_PUTPAGES doesn't formally require it.

[kib]

sys/kern/vfs_vnops.c

2568

The call chain is vm_object_page_clean()->vm_object_page_collect_flush()->vm_pageout_flush()->vm_pager_put_pages()->vnode_pager_putpages()->VOP_PUTPAGES()->vnode_pager_generic_putpages()->VOP_WRITE()

VOP_PUTPAGES() only requires locked vnode because some filesystems (zfs) only required read-locked vnode for write. But for UFS, VOP_WRITE() must be provided with exclusively-locked vp.

[asomers]

LGTM and it fixes the tests for UFS, though I'm not an expert in this area. The problem with fusefs has a different cause; I'll discuss on bugzilla.