Add a vop_stdioctl() that does the trivial algorithm for FIOSEEKDATA/FIOSEEKHOLE

[rmacklem]

Summary

For file systems that do not implement holes, Linux provides the trivial implementation
for FIOSEEKDATA/FIOSEEKHOLE that returns the offset for FIOSEEKDATA and offset == file_size
for FIOSEEKHOLE.
Also, this seemed to be the preferred choice when discussed on freebsd-current@ w.r.t. what
to do when file systems don't implement VOP_IOCTL().

This patch implements this by adding a vop_stdioctl() that does it.

Test Plan

Tested using an NFS mount (which does not support VOP_IOCTL()) and a test program that does
lseek(SEEK_DATA) and lseek(SEEK_HOLE) for various offsets.
Since it is a VOP_IOCTL(), it follows the convention of returning ENOTTY for certain failures, even
though lseek() is not documented as returning that error (and ENOTTY is not listed as an errno for POSIX).

[kib]

kern/vfs_default.c

1155 ↗

(On Diff #60925)

I think you may check v_type before calling VOP_GETATTR()

1166 ↗

(On Diff #60925)

Add break; for consistency.

[rmacklem]

This version of the patch has the changes suggested by kib@ incorporated in it.
The only semantic change is that it now returns whatever error VOP_GETATTR()
returns if VOP_GETATTR() fails. This is consistent with the behaviour of vn_bmap_seekhole().