generic: add fscaps regression test

generic: add fscaps regression test

Add a test to verify that setting a v3 fscap from an idmapped mount
works as expected. This and other related use-cases were regressed by
commit [1] which was reverted in [2] and the proper fix merged right
before v5.12 was released in [3].

[1]: commit 3b0c2d3eaa83 ("Revert 95ebabde382c ("capabilities: Don't allow writing ambiguous v3 file capabilities")")
[2]: commit 95ebabde382c ("capabilities: Don't allow writing ambiguous v3 file capabilities")
[3]: commit db2e718a4798 ("capabilities: require CAP_SETFCAP to map uid 0")

Cc: fstests@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>