RFC: SEEK_HOLE/SEEK_DATA common implementation

[Konstantin Belousov]

Hello,
Please find at
http://people.freebsd.org/~kib/misc/seek_hole.1.patch
a prototype generic implementation of the SEEK_HOLE/SEEK_DATA lseek(2)
whence command for any filesystem that properly supports bmap.

I was unable to find any test suite for the feature, and the only more
or less reasonable case I found was from lklm, extracted at
http://people.freebsd.org/~kib/misc/seek_hole.c
The block for file with hole at the end is commented out because
UFS never puts holes at EOF. The block_size is tuned for default block
size on UFS on recent FreeBSD.

Filesystem-specific implementations could greatly increase the performance
of the call, because the presented implementation does linear search
through the blocks until hole/data boundary is found. E.g., for UFS,
the fast tree-like structure of indirect blocks can be traversed to
point at the next boundary. But this implementation is generic.

Please comment.
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