Optimize lseek(SEEK_DATA) on UFS.

The old implementation, at the VFS layer, would map the entire range of
logical blocks between the starting offset and the first data block
following that offset. With large sparse files this is very
inefficient. The VFS currently doesn't provide an interface to improve
upon the current implementation in a generic way.

Add ufs_bmap_seekdata(), which uses the obvious algorithm of scanning
indirect blocks to look for data blocks. Use it instead of
vn_bmap_seekhole() to implement SEEK_DATA.

Reviewed by: kib, mckusick
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19598