Allow fscache to work on BTRFS

The following three patches allow fs to "cachefiles" in a BTRFS filesystem.

The first is a minor cleanup to cachefiles.
The second is the main change - it teaches cachefile to use lseek(SEEK_DATA) to find allocated blocks in a file, rather than bmap. The third patch simply enables this for btrfs.

Thanks,
NeilBrown

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NeilBrown (3):
cachefiles: perform test on s_blocksize when opening cache file. fscache/cachefiles: optionally use SEEK_DATA instead of ->bmap.
btrfs: set FS_SUPPORTS_SEEK_HOLE flag.

 fs/btrfs/super.c      |    3 +
 fs/cachefiles/namei.c |   13 ++++-
 fs/cachefiles/rdwr.c  |  125 ++++++++++++++++++++++++++++++-------------------
 include/linux/fs.h    |    1 
 4 files changed, 88 insertions(+), 54 deletions(-)

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