generic/301: skip extent count check on btrfs

generic/301: skip extent count check on btrfs

The 2/3 fragmentation threshold was designed for XFS's cowextsize
mechanism and is not applicable to btrfs. After commit 0dc118b3c327
("btrfs: be less aggressive with metadata overcommit when we can do
full flushing"), btrfs commits transactions more frequently during
writeback, reducing the extent allocator's opportunity to coalesce
adjacent CoW extents. This causes the extent count to sometimes cross
the threshold, making the test flaky on btrfs.

Skip only the extent count assertion for btrfs while keeping the CoW
write + remount + md5sum data integrity checks, which remain useful.

Signed-off-by: Leo Martins <loemra.dev@gmail.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>