Implement physical rewrites

Based on previous commit this implements `zfs rewrite -P` flag,
making ZFS to keep blocks logical birth times while rewriting
files.  It should exclude the rewritten blocks from incremental
sends, snapshot diffs, etc.  Snapshots space usage same time will
reflect the additional space usage from newly allocated blocks.

Since this begins to use new "rewrite" flag in the block pointers,
this commit introduces a new read-compatible per-dataset feature
physical_rewrite.  It must be enabled for the command to not fail,
it is activated on first use and deactivated on deletion of the
last affected dataset.

Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:  Alexander Motin <alexander.motin@TrueNAS.com>
Closes #17565

man/man8/zfs-rewrite.8

@@ -31,7 +31,7 @@
 .Sh SYNOPSIS
 .Nm zfs
 .Cm rewrite
-.Oo Fl rvx Ns Oc
+.Oo Fl Prvx Ns Oc
 .Op Fl l Ar length
 .Op Fl o Ar offset
 .Ar file Ns | Ns Ar directory Ns …
@@ -43,6 +43,15 @@ as is without modification at a new location and possibly with new
 properties, such as checksum, compression, dedup, copies, etc,
 as if they were atomically read and written back.
 .Bl -tag -width "-r"
+.It Fl P
+Perform physical rewrite, preserving logical birth time of blocks.
+By default, rewrite updates logical birth times, making blocks appear
+as modified in snapshots and incremental send streams.
+Physical rewrite preserves logical birth times, avoiding unnecessary
+inclusion in incremental streams.
+Physical rewrite requires the
+.Sy physical_rewrite
+feature to be enabled on the pool.
 .It Fl l Ar length
 Rewrite at most this number of bytes.
 .It Fl o Ar offset
@@ -60,17 +69,22 @@ same as some property changes may increase pool space usage.
 Holes that were never written or were previously zero-compressed are
 not rewritten and will remain holes even if compression is disabled.
 .Pp
-Rewritten blocks will be seen as modified in next snapshot and as such
-included into the incremental
-.Nm zfs Cm send
-stream.
-.Pp
 If a
 .Fl l
 or
 .Fl o
 value request a rewrite to regions past the end of the file, then those
 regions are silently ignored, and no error is reported.
+.Pp
+By default, rewritten blocks update their logical birth time,
+meaning they will be included in incremental
+.Nm zfs Cm send
+streams as modified data.
+When the
+.Fl P
+flag is used, rewritten blocks preserve their logical birth time, since
+there are no user data changes.
 .
 .Sh SEE ALSO
-.Xr zfsprops 7
+.Xr zfsprops 7 ,
+.Xr zpool-features 7